For a green horse, a generous release is normally the best way to go. You don't want to catch him in the mouth and give him a bad experience. A generous release also lets him learn to use his neck to help balance himself.
For a crest release, you don't want your hands floating above the neck. Just think extend arms forward, plant knuckles into the neck just below the mane. Rein length short enough that you have control on the flat, but long enough that extending your arms forward at jump time puts a nice loop in the reins.
As for seat on approach, that depends. Teaching a horse to jump is done mostly through grids since single fences can be difficult for green horses to figure out. Grids have nice set distances and can be set up in succession by adding elements as the horse masters the previous ones. Trot in, canter out.
For single fences, the answer is, it depends. If you have a forward horse who wants to jump, a light seat is probably fine. Stay out of his way and let him jump. For a more timid horse, you will need to sit and drive him to the fence in the proper tempo. Normally horses will be timid the first few fences and then decide it's fun and become more forward, so your seat on approach changes to fit that.
Hope that helps.
You just have to see your distance...you don't have to like it.