The experience of 1st jhana can vary widely depending on how steady the mind is and what the object is. But for most practical purposes, learning to take mindfulness of breathing to evoke rapture is a go-to method, because it can be developed even outside of formal meditation. Other methods like mantra can be especially useful for evoking rapture and entering the first jhana.
In general terms, the first jhana state of being with a main element of physical pleasure felt throughout the body. It’s a pleasure unlike any kind of sensory pleasure, it has a pure feeling to it that feels nourishing for your body and mind. It is very pleasurable and soothing for the mind.
A useful comparison to make is the comparison of sex to first jhana. The pleasure of rapture is reminiscent of sex and feels like an orgasm throughout your body. As if every cell of your body was a little pleasure balloon and the balloons are popping and forming and popping. It can be cultivated to be very intense, but regardless of the depth, there is this very salient sense of being in a different and “better” state of consciousness.
The mental “posture” that keeps you in the first jhana and the dynamic happening is similar to having an extended orgasm, and in fact learning the latter is informative for the former and two skills are related. It feels like riding a wave and being aware of what’s happening. Jhana is different than sex because it is not based in sensory pleasure external to the mind. But it can be compared to sex because of the mental orientation toward phenomena and the sense of “riding a wave”.
The essential way to use jhana to realize insight, is to maintain a sense of discernment of what’s going on in the mind at a higher, metacognitive level, and observing the activities and noticing the effect of the state on the mind and what is different in jhana vs. not.
In the first jhana, there is the physical pleasure, and the mind tends to come to an equanimity towards it. It is ok if it leaves and actually a more peaceful state would not have all this activity and intensity. It’s a little bit tiring having to keep maintaining attention on the rapture, and it is in close proximity to access concentration, where there may be rapture and other factors of awakening present, but the sense of flow and entering a pervasive state are not present (category III concentration). This falling out of first jhana into CCC is normal, and it’s helpful to watch the mind when the rapture goes away, or before the jhana has stabilized, especially when learning, the falling out of jhana shows directly clinging and craving. You see the mental grasping that happens when the pleasant state falls out. And learning to be equanimous with this dynamic is essential to stabilize 1st jhana. That dynamic is a more refined and clear way of noticing the links of craving and clinging, which are very transient mind moments arising and passing rapidly.
After gaining some skill in entering the 1st jhana and stabilizing it, one can simply allow the mind to rise to an intuitively more refined way of seeing- the mental joyfulness and happiness take center stage. There is a sense of giddy happiness in the background of 1st jhana, and one way to enter 2nd jhana is to take that as the main focus and let go of applied and sustained attention. The mind is now collected due to mental unification, and the flow state takes a distinctly more effortless tone. The mind can bask in the joyful happiness and it’s effortless.