Start by making a note of each topic that may come up in your exam. You can find these in your textbook or revision guide. Let me know if you need help finding the list of topics that your exam board uses. 

Then, divide up your time so you have time to revise each unit. For each unit, read through your textbook or revision guide to remind yourself of the topic. Answer some questions from your textbook, trying to do them on your own but looking through your textbook if you need to. 

When you have gone through each topic, try to do a past paper - you can look through your notes if you need to! 

Then try to do a past paper without your notes. 

Finally you can try to do a past paper without your notes and under timed conditions. Don't feel discouraged if this is hard at first! Working under timed conditions takes practice :)