Let us begin by saying what this preparation is not:
It is not sitting in meditation rather like a runner or footballer might do as a warming up exercise.
It is not intellectually chewing over mentally what is to be accomplished like a rabbit until the contents are mashed into nothing at all for digestion.
It requires imagination that is precise and directed. One sits at any time in any place where there is sufficient tranquility and imagines the natural unfolding of the meditation from beginning to end. This is useful as it conditions the Becoming of Consciousness to correctly establish the meditation by:
Breathing In and then In and Out with mindfulness; then, realizing the whole body (in Chan we call this Defensive Qi), establish the calming of all body elements.
Dwelling or Abiding then in that state, doing nothing with the cognitive mind at all, imagine the arrival of the two essential experiences of Joy (piti) and Happiness.
In the preparation imagine that the previous mind- prepared Concentration or Contemplation Devices enter without a cognitive impulse. You need no details, for you are just imagining that this is all happening perfectly.
It is rather like a "high jump" specialist who rocks forward and backward before his run imagining every detail of the jump without actually doing it, or a golfer who imagines fully and perfectly the stroke he is about to make.
Imagine the Device entering as a cognitive construct, the last conceptualization, and then transforming it into a Cognitive Experience.
Then imagine the traquilization of these Cognitive Experiences, which are thought elements. They dissapear and though you must imagine that you are sitting in an almost empty space, that Subliminal Experiences are taking place, uninterrupted by impediments.
This results in a subliminal Realizing the Mind, Gladdening the Mind, Concentrating the Mind and finally Releasing the Mind.
In the meditation this happens and your consciousness knows nothing at all about what is happening. This is where the meditation bears fruit and you must imagine that this is happening.
Four which are the Essential Discernments:
Discerning Cessation of Asavas (defilements)
Discerning Entire Freedom