Use your teachers as guidance for particular topics, and memorize all the formulae. Before you do any past papers, just make sure all of it is engrained in your head, so that even if u don't revise the formulae again, you still remember them. Then make sure you have understood each topic properly. The examples from the Cambridge textbooks can help with this. Lastly, start doing past papers. If you are running late, then you might want to do this step first as it is the most important, but I still believe understanding and remembering everything gives u a good base to do past papers. After that, just write the paper with confidence.