How to complete the first Jhana?

The answers here are so much more different than what Ajahn Brahm teaches. His technique is that of let-go. The answers here are focused on concentration on bliss. I have never entered jhnana but intutively I feel Ajahn Brahm is right. Refer to his book The Jhanas, or google Ajahn Brahm Jhnanas -- his technique guide will come up.

Samatha meditation teacher and book author Leigh Brasington, who is a student of Ayya Khema and Jack Kornfield, wrote in his guide "Entering the Jhanas":

The path to entering the jhanas begins with what is called access concentration: being fully with the object of meditation and not becoming distracted even if there are wispy background thoughts. If your practice is anapanasati—mindfulness of breathing—you may recognize access concentration when the breath becomes very subtle; instead of a normal breath, you notice your breath has become very shallow. It may even seem that you’ve stopped breathing altogether. These are signs that you’ve likely arrived at access concentration. If the breath gets very shallow, and particularly if it feels like you’ve stopped breathing, the natural thing to do is to take a nice deep breath and get it going again. Wrong! This will tend to weaken your concentration. By taking that nice deep breath, you decrease the strength of your concentration. Just stay with that shallow breathing. It’s okay. You don’t need a lot of oxygen when you are very quiet both physically and mentally.

If the breath gets very, very subtle, instead of taking a deep breath, shift your attention away from the breath to a pleasant sensation. This is key. You notice the breath until you arrive at and sustain access concentration, then you let go of the breath and shift your attention to a pleasant sensation, preferably a physical one. There is not much point in trying to notice the breath that has gotten extremely subtle or has disappeared completely—there’s nothing left to notice.

The first question that may arise when I say, “Shift your attention to a pleasant sensation” may be “What pleasant sensation?” Well, it turns out that when you get to access concentration, the odds are quite strong that, someplace in your physical being, there will be a pleasant sensation. Look at most any statue of the Buddha—he has a faint smile on his face. That is not just for artistic purposes; it is there for teaching purposes. Smile when you meditate, because once you reach access concentration, you only have to shift your attention one inch to find a pleasant sensation.

Pleasant sensations can occur pretty much anywhere. The most common place that people find pleasant sensations when they’ve established access concentration is in the hands. When you meditate, you want to put your hands in a comfortable position in which you can just leave them. The traditional posture is one hand holding the other, with the thumbs lightly touching. But you can also put your hands in all sorts of other positions—just place them however it appeals to you. After you’ve been in access concentration “long enough,” if you notice that there’s a pleasant feeling in the hands, drop the attention on the breath and focus entirely on the pleasantness of that sensation.

Another common place where people find a pleasant sensation is in the heart center, particularly if they’re using metta, or loving-kindness, meditation as the access method. Just shift your attention to the pleasantness of that sensation. Other places people find pleasant sensations could include the third eye, the top of the head, or the shoulders. It does not matter where the pleasant sensation manifests; what matters is that there is a pleasant sensation and you’re able to put your attention on it and—now here comes the really hard part—do nothing else.

It’s important to let go of the breath when you make the shift to the pleasant sensation. The breath (or other meditation object) is the key to get you in—”in” being synonymous with establishing strong enough access concentration. When you come home from work, you pull out your key, you open the door to your home, and you go in. You don’t then wander around with the key still in your hand—you put it back in your pocket or purse or on some table. You’re not cooking dinner or watching TV with the key still in your hand. The key has done its job, and you let it go. It’s exactly the same with the breath or other meditation object. Totally let go of it, and focus entirely on the pleasant sensation. Of course, this is easier said than done—you’ve struggled for a long time to stay locked onto the breath, and now that you’ve finally managed to do so, the first thing you are told is to stop doing that. But that’s the way it is. If you want to experience jhanas, it’s going to be necessary to give yourself to fully enjoying the pleasantness of the pleasant sensation.

Once you’ve found the pleasant sensation, you fully shift your attention to it. If you can do that, the sensation will begin to grow in intensity; it will become stronger. This will not happen in a linear way. At first, nothing happens. Then it’ll grow a little bit and then hang out and grow a little bit more. And then eventually, it will suddenly take off and take you into what is obviously an altered state of consciousness.