Well I'd think about what you'd least like to have a question on, and start revising that. Read through the revision guide on that topic first, then make notes, then get friends/family to test you on it. If it's a chemical equations type question like titrations then do some practice questions until you can do them easily. After that, you should know the topic really well. Do that again with your 2nd least favourite topic and so on. Hope this helps and good luck :)
Testing yourself with a past paper is a good way to go, or you could start from the very (and I mean VERY) beginning of the topic.
eg. topic 1 of unit 1. then gradually move to topic by topic so that everything piece up together as a whole. or if u feel topic 1 is not a problem to you, then have a look at it and move straight to topic 2.
The thing I dislike about chemistry is that the questions in the exams are always harder than the material taught. I suggest doing a past paper to get a taste for it. See how you do. and make a note of the topics you stuggled with.