Depends on what unit you're doing. For me (in unit 1) I made several quick revision cards with the basic things, which then in my mind I would elaborate further. But there are several ways of revising; you just need to find the one that is best for you.
For example, doing spider diagrams, highlighting the text book (if you can!), making flash cards or producing MP3's out of revision documents can all help. All the advice I can give is to try and start as early as possible and try and find the best technique for yourself, since not everyone uses the same one!
just revise if unit 1- revsie 2 units for unit 2- revise 3 units properly revise it leave 2 weeks for each unit and you'll get an A. it works trust me. in AS it is so easy to get an A dont ruin ur chances its really baby work.