Start off by listening in lesson, staying focused this will make revision a lot easier.
Get a textbook and go through the whole unit, taking EVERY note possible (short simple and neat). Then create notes from your notes, highlighting everything you're not 100% certain with, then create some more notes from those. Repeat until you are left with basically nothing. After this, get as many past exam papers along with mark schemes and go through them. If you know how to do a question just SKIP IT. When you get stuck go back to your notes so you can relearn it again. If you do this, you will fly through the exam.
If you are really really easily distracted like me, it may sound stupid but actually go somewhere quiet like a library and do the work there.
Plus this method worked for me, I literally slept in all my lessons (no joke) and learned virtually nothing throughout the year. So i had to take it upon myself to learn it all by my own. This process took about 15 hours per unit altogether (But i am pretty slow). If you are a hardcore worker then you could literally learn the whole unit inside out in a weekend.
Good luck.