I would believe not even one answer unless one makes the experience for oneself. Be even skeptical that Nirvana exists, for there is no real evidence unless one is experiencing it. Just because the Buddha said it "exists" doesn't make it so. All people answer as if they are enlightened or made the experience .. hell as if they are expert on this field but they are not. If nirvana really exists it must be possible to see it under a fmri. Why? If one really abandoned greed, hatred and delusion one can expose the "enlightened" person with evocative material to see whether any greed, hatred or delusion appears in the brain. I'm positive that there will.

This may seem lengthy to some, but it simply is. The comment above this one is correct. There are many approaches, this is the personal approach my mind went through to be able to achieve it. My body and mind achieved nirvana completely by accident when experimenting with meditation at age 17. This was brief, lasting somewhere between a half an hour and a few hours. It sought it again and failed and wrote it off as a delusion until later. See, the approach was wrong, the WANT of nirvana itself distracted my body and mind from achieving it. The Buddhist obviously have temples where monks go to achieve it, but it seems to take years for them. So how did this body achieve it by accident on it's first occasion meditating? Well, under had a very good teacher for one. My mind was also under the influence of an African tree ash that aids in meditation. The concept of achieving nirvana without seeking it seems terribly difficult/backwards, but really that in itself is the lesson. I really wanted to stress that point.

But overall, you must devoid your mind of "right" and "wrong". You must take and see things as they are, without an opinion of it. You must learn to let go of all desire. To transcend your own ego. There are many ways to coin it, but it is in fact a personal journey that must be achieved personally. For some people, this takes many many years. If you have trouble, that just means to need more practice, and you may need to apply the practice into your outside life. A teacher helps with all this. For meditation in general you will receive benefits without achieving nirvana. To properly meditate is the beginning. This body has found that having a focus/focal point for concentration helps. My mind uses breathing. In through nose, deep into the abdomen, out through the mouth, again, as deeply as possible. My mind focuses on this. Relaxing as much as possible. Then a thought crosses my mind. I take that thought, acknowledge it, and complete it. That's the process. The key for me is to not judge, to not even have an opinion on it, to simply see it for what it is. This seems to work for others as well, but I don't claim to be a mind reader in the sense that I know every step they took to achieve this state. There are many religions, nay schools of thought, that can give you more details, but truly if you understand one grain of salt, you understand all of them. One can go and study these things, the internet is a wonderful tool. There is no right and wrong. There simply is. Accept this, work at it, and the road never ends, achieving nirvana isn't a stopping point. Do it all for self-improvement. Do not confuse stubbornness for patience.

Also, I remember the exact thought that was the focal point for turning my mind into proper thought. An old saying of proper thought, that I came across whilst reading the Tao of Jeet Kun Do by Bruce Lee. I don't know if it'll help you, I don't offer a cure all. But again, this is my personal journey. "Empty your cup, so it may be filled."

In order to achieve Nirvana (the Theravadin version of Enlightenment) you need to do a lot of mindfulness meditation and have a lot of good karma. It is not possible to understand Nirvana in terms of modern psychology and philosophy, because Buddhist psychology is very very difficult to understand. Fortunately, you do not need to understand Buddhist psychology to achieve Enlightenment. These days, suffering is mostly caused by tragic adaptations to an abusive or dysfunctional family background, causing problems in self-love and finding love. I have written a book on mindfulness meditation, if you want to read about my 50 years of experience with traditional mindfulness meditation.

Recite & memorize kītāgirisutta:

"Monks, I do not say that the attainment of gnosis is all at once. Rather, the attainment of gnosis is after respectively training, respectively action, respectively practice. And how is there the attainment of gnosis after respectively training, respectively action, respectively practice?

1. There is the case where, when conviction has arisen, one visits [a teacher].
2. Having visited, one request to be his attendant (see vinaya mahāvagga mahākhandhaka, and CH. III TAKING A MEDITATION SUBJECT of Visuddhimagga)
3. Having been attendant, one listen carefully.
4. Having listened carefully, one hears the Dhamma.
5. Having heard the Dhamma, one memorizes it (dhatā=sutadharo).
6. Having remembered by memorizing, one ponders the meaning of the teachings.
7. Having pondered the meaning, one comes to an agreement through pondering the teachings.
8. There being an agreement through pondering the teachings, wish-to-do arises.
9. When wishing-to-do has arisen, one exerts.
10. When one exerted (an effort), one meditates insight.
11. Having meditated insight, one dedicates his life.
12. Having dedicated his life, one realizes with the mind (discernment's body) the ultimate truth and, having penetrated, enlightened, it with discernment, sees it.

"Now, monks, there hasn't been that conviction, there hasn't been that visiting, there hasn't been that attending ... that listening carefully ... that hearing of the Dhamma ... that memorizing ... that penetration of the meaning of the teachings ... that agreement through pondering the teachings ... that wishing-to-do ... that effort ... that meditating insight ... that dedicating his life. You have lost the way, monks. You have gone the wrong way, monks. How far have you strayed, foolish men (moghapurisa), from this Dhamma & Discipline!

I have edited many part of the translated sutta follow to many pali of many sutta and commentaries. I wish the editing makes easier for the reader to connect the sequence of steps.

"The Buddha once said, let an observant person come who is honest and no deceiver, and he would teach that person the Dhamma. Those are his prerequisites for anyone who was going to practice and get results. If you’re not honest with yourself, you’ll start to deny things that are arising and passing away in the mind. And if you’re not observant, it all goes right past you.

This means that the Dhamma is something special. The discernment of the Dhamma is something special. It’s not something you can figure out by reading books and being clever. It requires good qualities of the heart, and in particular this quality of honesty, which is basic to all the other good qualities you need. So make sure that honesty enters into the mix as well. That’s how the discernment becomes penetrating, as the Buddha says, leading to the right ending of suffering and stress."

~ Thanissaro Bhikkhu "Developing Discernment" https://www.dhammatalks.org/Archive/Writings/CrossIndexed/Published/Meditations9/090804_Developing_Discernment.pdf

I believe that all Buddhist "sects" would say that there is a path to nirvana. In deed this is the "fourth noble truth", though different "sects" interpret it differently, or at least claim that other "sects" have a different understanding of it. For your purposes, maybe that's what the long history of doctrinal debate is actually about.

I believe that all Buddhists claim that meditation is a prerequisite to nirvana, though that seems to take different forms, with different "sects" vying over e.g. the terms "sudden" and "gradual", and with different instructions and communities.

I've put 'sect' in quotes because I'm going to side with what I understand from soto zen, which I think stresses that we "don’t practice zazen in order to get enlightened, we practice being enlightened as we practice zazen". So the f*kanzazengi by Dogen says:

The zazen I speak of is not learning meditation. It is simply the Dharma gate of repose and bliss, the practice-realization of totally culminated enlightenment. It is the manifestation of ultimate reality... If you concentrate your effort single-mindedly, that in itself is negotiating the Way

Not sure about many religions. I think Hinduism and Jainism have concepts of Nirvana. But they are different from the Buddhist teaching of Nibbana. Anyways, it's so simple in Buddhism. You only need to eliminate craving to attain what Buddhists mean by 'Nibbana'. Read about the "Four Noble Truths".

Nirvana can defined as a peaceful state of mind that is free from craving, anger and mental suffering.

So answer for, What is required to achieve Nirvana is eliminate all the above things.

Nirvana is not a one-time switch... You're in Samsara - BOOM! - you're in Nirvana and for good! It's not like that.

Nirvana is a momentary experience of freedom and peace. It can ONLY be such as there is nothing else existent but this moment. To realize such peace and freedom, it's helpful to loosen up one's attachment to desirable conditions and rejection of undesirable conditions. In other words, to loosen up your dependence on conditions overall when it comes to happiness and peace.

Through meditation practice and discipline in everyday life, it will become easier and easier to let go of our expectations, assumptions and demands that life go our way. That's not to say we don't make an effort to have favorable conditions in our lives... Not at all. That would be very anti-life and against our basic nature. We all enjoy good things and rightfully so.

But when our desire for good things becomes constant craving and strong attachment to those things, we're screwed because all good things come to an end (and are replaced by "bad things" like loss, old age, sickness and - gulp! - death). There is no peace or happiness in such craving and attachment. And there's no lack of peace or unhappiness in "bad" conditions either. But our normal approach is to loop endlessly through these cycles of craving, clinging and disappointment. It's very tiring and painful.

"Nirvana" is the momentary experience of being free of this endless cycle. It's that simple. And the more you experience this freedom, the less you will care about the changing conditions of your life and be able to flow with all of it. Yet, you are never finished with developing this capacity as it goes very deep.

In Theravadan tradition you need to gain Insight through Vipassana meditation in order to attain Nirvana. That's all! Everytthing else just makes the path to Enlightenment easier or has other benefits like Samatha meditation.

Nirvana is not an achievement! According to the Buddha there is no path to it because then nirvana becomes an effect of taking that particular path. Also, no practice will lead you to nirvana. Because again, nirvana becomes the effect of the practice. If nirvana is an effect it's not the end of suffering is it? Buddha's only advice was to fully investigate, understand the cause of suffering. According to him understanding of the cause is the ending of suffering. Paths and practices were introduced by monks(not the Buddha) for their survival, existence!

I am reading a book called What the Buddha Taught by Dr. Walpola Sri Rahula.

Noble Eightfold Path

The Fourth Noble Truth is 'The Path' (Magga) leading to the cessation of Dukkha (defined in the First Noble Truth as basically anything conditioned or in other words part of dualism; things that are not everlasting)

See other answers that already list the eight divisions/categories.

Five Hinderances

Ponder these.

1. lustful desires (kāmacchanda)
2. ill-will, hatred, or anger (vyāpāda)
3. torpor and languor (thīna-middha)
4. restlessness and worry (uddhacca-kukkucca)
5. sceptical doubts (vicikicchā)

Seven Factors of Enlightenment

Meditate on these.

1. Mindfulness (sati) i.e. to be aware and mindful in all activities and movements both physical and mental.
2. Investigation and research into the various problems of doctrine (dhamma-vicaya). Included here are all our religious, ethical, and philosophical studies, reading, researches, discussions, conversations, even attending lectures relating to such doctrinal subjects.
3. Energy (viriya), to work with determination till the end.
4. Joy (pīti), the quality quite contrary to the pessimistic, gloomy, or melancholic attitude of mind.
5. Relaxation (passaddhi) of both body and mind. One should not be stiff physically or mentally.
6. Concentration (samādhi).
7. Equanimity (upekkhā) i.e. to be able to face life in all its vicissitudes with calm of mind, tranquillity, without disturbance.

To cultivate these qualities the most essential thing is a genuine wish, will, or inclination. [...] One may also meditate on such subjects as the Five Aggregates investigating the question "What is being?" or "What is it that is called I?", or on the Four Noble Truths. Study and investigation of those subjects constitute this fourth form of meditation, which leads to the realization of Ultimate Truth.

Excerpt about the feeling 'I AM'

It is the vague feeling 'I AM' that creates the idea of self which has no corresponding reality, and to see this truth is to realize Nirvāna, which is not very easy. In the Samyutta-nikāya there is an enlightening conversation on this point between a bhikkhu named Khemaka and a group of bhikkhus.

These bhikkhus ask Khemaka whether he sees in the Five Aggregates any self or anything pertaining to a self. Khemaka relies 'No'. Then the bhikkhus say that, if so, he should be an Arahant free from all impurities. But Khemaka confesses that though he does not find in the Five Aggregates a self, or anything pertaining to self, 'I am not an Arahant free from all impurities.' O friends, with regard to the Five Aggregates of Attachment, I have a feeling "I AM", but I do not clearly see "This is I AM".' Then Khemaka explains that what he calls 'I AM' is neither matter, sensation, perception, mental formations, nor consciousness, nor anything without them. But he has the feeling 'I AM' with regard to the Five Aggregates, though he could not see clearly 'This is I AM'.

He says it is like the smell of a flower: it is neither the smell of the petals, nor the color, not of the pollen, but the smell of the flower. Khemata further explains that even a person who has attained the early stages of realization still retains this feeling 'I AM'. But later on, when he progresses further, this feeling of 'I AM' altogether disappears, just as the chemical smell of a freshly washed cloth disappears after a time when it is kept in a box.

This discussion was so useful and enlightening to them that at the end of it, the text says, all of them, including Khemata himself, became Arahants free from all impurities, thus finally getting rid of 'I AM'.

In Buddhism you need to follow the Noble Eight Fold path to attain nirvana. It is further divided into three parts for simplicity - Sila (morality), Samadhi (mental culture) and Panna ( wisdom).

The noble eight fold path is as below : Sila (Morality): 1) right speech 2) right action 3) right livelihood Samadhi (mental culture): 4) right awareness 5) right concentration 6) right efforts Panna (wisdom) : 7) right resolve 8) right view

In whichever teaching the components of Noble Eight Fold path is found there you will find source of Nirvana. So you can follow any sect you prefer. But right view is what differs in Buddhism which takes you in right direction. Understanding Buddhism's (right) view of rebirth, karma and three realities of life (impermanence, no soul and suffering) and dependent origination can help understand Nirvana better.