What were the Original Methods of Meditation and their Instructions as Expounded by the Buddha?

The word meditation doesn't have an analog in pali discourses.

The way samatha & vipassana are spoken of, likewise, doesn't align with contemporary practices.

Unfortunately one has to read a lot of discourses to get a good idea of how people were training on & off the cushion and how they talked about it.

If you want a bunch of excerpts from pali sutta & theravadin abhidhamma + minimal commentary, i have a doc here, only the first half of it will be interesting to you as a cross reference of the main meditation instruction in the discourses.

It's a 24/7 kind of training. Here are the brief expositions;

"Kesi, I train a tamable person [sometimes] with gentleness, [sometimes] with harshness, [sometimes] with both gentleness & harshness.

"In using gentleness, [I teach:] 'Such is good bodily conduct. Such is the result of good bodily conduct. Such is good verbal conduct. Such is the result of good verbal conduct. Such is good mental conduct. Such is the result of good mental conduct. Such are the devas. Such are human beings.'

"In using harshness, [I teach:] 'Such is bodily misconduct. Such is the result of bodily misconduct. Such is verbal misconduct. Such is the result of verbal misconduct. Such is mental misconduct. Such is the result of mental misconduct. Such is hell. Such is the animal womb. Such the realm of the hungry shades.'

"In using gentleness & harshness, [I teach:] 'Such is good bodily conduct. Such is the result of good bodily conduct. Such is bodily misconduct. Such is the result of bodily misconduct. Such is good verbal conduct. Such is the result of good verbal conduct. Such is verbal misconduct. Such is the result of verbal misconduct. Such is good mental conduct. Such is the result of good mental conduct. Such is mental misconduct. Such is the result of mental misconduct. Such are the devas. Such are human beings. Such is hell. Such is the animal womb. Such the realm of the hungry shades.'"
AN 4.111

“Master Assaji how does Gotama the Contemplative generally instruct his disciples?”

“Aggivessana, Gotama the Rightly Self-awakened one instructs his disciples, in a general way, as follows: ‘Form is impermanent, feeling is impermanent, perception is impermanent, mental fabrications are impermanent, consciousness is impermanent. Being impermanent, form is not-self, feelings are not-self, perceptions are not-self, mental fabrications are not-self, and consciousness is not-self. All impermanent phenomena is not-self. In short, the Five Clinging-Aggregates are impermanent and so not-self.’ Aggivessana, this is the general way that Gotama the Rightly Self-awakened one instructs his disciples.”
Cula Saccaka Sutta

When, on observing that the monk is purified with regard to qualities based on delusion, he places conviction in him. With the arising of conviction, he visits him & grows close to him. Growing close to him, he lends ear. Lending ear, he hears the Dhamma. Hearing the Dhamma, he remembers it. Remembering it, he penetrates the meaning of those dhammas. Penetrating the meaning, he comes to an agreement through pondering those dhammas. There being an agreement through pondering those dhammas, desire arises. With the arising of desire, he becomes willing. Willing, he contemplates (lit: "weighs," "compares"). Contemplating, he makes an exertion. Exerting himself, he both realizes the ultimate meaning of the truth with his body and sees by penetrating it with discernment.

"To this extent, Bharadvaja, there is an awakening to the truth. To this extent one awakens to the truth. I describe this as an awakening to the truth. But it is not yet the final attainment of the truth.

"Yes, Master Gotama, to this extent there is an awakening to the truth. To this extent one awakens to the truth. We regard this as an awakening to the truth. But to what extent is there the final attainment of the truth? To what extent does one finally attain the truth? We ask Master Gotama about the final attainment of the truth."

"The cultivation, development, & pursuit of those very same qualities: to this extent, Bharadvaja, there is the final attainment of the truth. To this extent one finally attains the truth. I describe this as the final attainment of the truth."
MN 95

"May the Blessed One teach me the Dhamma in brief! May the One Well-gone teach me the Dhamma in brief! It may well be that I will understand the Blessed One's words. It may well be that I will become an heir to the Blessed One's words."

"Then, monk, you should train yourself thus: 'My mind will be established inwardly, well-composed. No evil, unskillful qualities, once they have arisen, will remain consuming the mind.' That's how you should train yourself.

"Then you should train yourself thus: 'Good-will, as my awareness-release, will be developed, pursued, handed the reins and taken as a basis, given a grounding, steadied, consolidated, & well-undertaken.' That's how you should train yourself. When you have developed this concentration in this way, you should develop this concentration with directed thought & evaluation, you should develop it with no directed thought & a modicum of evaluation, you should develop it with no directed thought & no evaluation, you should develop it accompanied by rapture... not accompanied by rapture... endowed with a sense of enjoyment; you should develop it endowed with equanimity.

"When this concentration is thus developed, thus well-developed by you, you should then train yourself thus: 'Compassion, as my awareness-release... Appreciation, as my awareness-release... Equanimity, as my awareness-release, will be developed, pursued, handed the reins and taken as a basis, given a grounding, steadied, consolidated, & well-undertaken.' That's how you should train yourself. When you have developed this concentration in this way, you should develop this concentration with directed thought & evaluation, you should develop it with no directed thought & a modicum of evaluation, you should develop it with no directed thought & no evaluation, you should develop it accompanied by rapture... not accompanied by rapture... endowed with a sense of enjoyment; you should develop it endowed with equanimity.

"When this concentration is thus developed, thus well-developed by you, you should then train yourself thus: 'I will remain focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world.' That's how you should train yourself. When you have developed this concentration in this way, you should develop this concentration with directed thought & evaluation, you should develop it with no directed thought & a modicum of evaluation, you should develop it with no directed thought & no evaluation, you should develop it accompanied by rapture... not accompanied by rapture... endowed with a sense of enjoyment; you should develop it endowed with equanimity.

"When this concentration is thus developed, thus well-developed by you, you should train yourself: 'I will remain focused on feelings in & of themselves... the mind in & of itself... mental qualities in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world.' That's how you should train yourself. When you have developed this concentration in this way, you should develop this concentration with directed thought & evaluation, you should develop it with no directed thought & a modicum of evaluation, you should develop it with no directed thought & no evaluation, you should develop it accompanied by rapture... not accompanied by rapture... endowed with a sense of enjoyment; you should develop it endowed with equanimity.

"When this concentration is thus developed, thus well-developed by you, then wherever you go, you will go in comfort. Wherever you stand, you will stand in comfort. Wherever you sit, you will sit in comfort. Wherever you lie down, you will lie down in comfort."
AN 8.63

Then, Bāhiya, you should train yourself thus: In reference to the seen, there will be only the seen. In reference to the heard, only the heard. In reference to the sensed, only the sensed. In reference to the cognized, only the cognized. That is how you should train yourself. When for you there will be only the seen in reference to the seen, only the heard in reference to the heard, only the sensed in reference to the sensed, only the cognized in reference to the cognized, then, Bāhiya, there is no you in connection with that. When there is no you in connection with that, there is no you there. When there is no you there, you are neither here nor yonder nor between the two. This, just this, is the end of stress."

Through hearing this brief explanation of the Dhamma from the Blessed One, the mind of Bāhiya of the Bark-cloth right then and there was released from effluents through lack of clinging/sustenance.
Ud 1.10

These are short expositions.

In detail one begins by reflecting on results one's thoughts & behavior in general. One separates thoughts into two categories as to conducive and not conducive to the goal which one envisions for oneself. Furthermore, one starts cultivating the good ones by giving attention and stilling the formation of distracting thoughts;

When, indeed, bhikkhus, evil unskillful thoughts due to reflection on an adventitious object are eliminated, when they disappear, and the mind stands firm, settles down, becomes unified and concentrated just within (his subject of meditation), through his reflection on an object connected with skill, through his pondering on the disadvantages of unskillful thoughts, his endeavoring to be without attentiveness and reflection as regards those thoughts or through his restraining, subduing, and beating down of the evil mind by the good mind with clenched teeth and tongue pressing on the palate, that bhikkhu is called a master of the paths along which thoughts travel. The thought he wants to think, that, he thinks; the thought he does not want to think, that, he does not think. He has cut down craving, removed the fetter, rightly mastered pride, and made an end of suffering."
MN 20

Assuming one has come to agreement with the Dhamma, one should reflect on one's actions before, during & after doing deeds, analyzing the results.

One should reflect on the fearfulness of the states of woe and give much attention to the danger in things. Cultivate self-preservation, kindness to oneself and this will make you a kind person to all beings because others are like yourself just different body & amount of delusion.

One should be one's own servant, protector, judge & advocate and let one's best knowledge dictate behavior.

One reflects much on one's circumstances, think thoughts of sympathy towards oneself, wanting oneself to succeed, think of ways to show compassion to yourself, a little kindness, restraining some bad behavior maybe or get rid of bad influence.

Think thoughts of appreciation much, count your blessings, don't ruminate on what you lack, think of how far you've come & etc. There are many other themes to develop, I have a list of course (see here).

Furthermore, one should remain always mindful of the body, the dhamma, the Buddha and the Sangha. What these mean exactly I won't get into here.

However, dwelling thus mindfully in dependence on strengths of a student, ardent & resolute, serious with perception of death acutely established, perception of unattractiveness well established, senses guarded, moderate with food, devoted to wakefulness, filled with learning, having a few duties, resting in solitude and with energy aroused, one will perceive great distinctions and quickly attain the final goal.

Here a few random excerpts on vipassana & samatha;

These two qualities have a share in clear knowing. Which two? Tranquillity (samatha) & insight (vipassana).

“When tranquillity is developed, what purpose does it serve? The mind is developed. And when the mind is developed, what purpose does it serve? Passion is abandoned.

“When insight is developed, what purpose does it serve? Discernment is developed. And when discernment is developed, what purpose does it serve? Ignorance is abandoned.

“Defiled by passion, the mind is not released. Defiled by ignorance, discernment does not develop. Thus from the fading of passion is there awareness-release. From the fading of ignorance is there discernment-release.”
AN 2.30

There is the case of the individual who has attained internal tranquillity of awareness (cetosamathassa), but not insight into phenomena through heightened discernment ( adhipañ­ñā­dhamma­vi­passa­nāya)

“The individual who has attained internal tranquillity of awareness, but not insight into phenomena through heightened discernment, should approach an individual who has attained insight into phenomena through heightened discernment and ask him: ‘How should fabrications be regarded? How should they be investigated? How should they be seen with insight?’ ...

“As for the individual who has attained insight into phenomena through heightened discernment, but not internal tranquillity of awareness, he should approach an individual who has attained internal tranquillity of awareness... and ask him, ‘How should the mind be steadied? How should it be made to settle down? How should it be unified? How should it be concentrated?’
AN 4.94

Basically, samatha is calming for impulse control and insight is associated with strategy, knowledge & understanding. If there is a lack of impulse control then one can't execute one's strategy and is distracted.

Links to the most important suttas on meditation here: https://lucid24.org/misc/raft/index.html (disclosure: my website, most of the sutta translations derived from B. sujato). You'll want to start with SN 46.3, it shows the causal sequence of how 7 awakening factors leads to samadhi. SN 54.3 is the basic version explaining 16 steps of breath meditation. SN 45.8 gives the definitions for noble eightfold path. Prior to attainment of jhanas, most of the time in meditation will be spent in right effort, explained in detail in AN 4.14

Remember that Buddhism was an oral tradition, so the instructions are terse, and you won't be able to get the full picture on reading through those suttas once or a few times. You'll need to recite and reflect on it frequently, and consult with knowledgable friends to connect the dots and see the whole picture.

Samatha and vipassana segregation is a late Theravada corruption. In the original suttas on meditation, those qualities are not developed independently of each other, but synergistically, holistically, organically.

edit, answering a comment: It's easy to misinterpret Yuganaddha Sutta that way (as segregating samatha and vipassana). But if you study carefully all the meditation suttas, you'll find that interpretation is not tenable, not to mention there are very few suttas that like that one period. In light of the collection of meditation suttas and connecting the dots, what Yuganaddha Sutta is actually saying is that some meditators tend to be stronger in samatha or vipassana, and on their path to arahantship they don't really strengthen the other factor until later stages. The 4th case of the sutta makes it especially clear. Both samatha and vipassana are weak to begin with, and later both are developed in tandem. So out of the 4 possible cases 50% of them involve developing both samatha and vipassana in tandem. The first two cases, where one is developed first while the other is lacking, is from inherent weakness in the meditator, not because the buddha ever recommended one factor to be developed first.