Which exam board are you doing?
I did edexcel and I was predicted a D in further, when it came to revising I made revision cards using the summaries at the end of each chapter in the textbooks and read them in my spare time. I did all of the past papers, and anything I got wrong I would add to a revision card. Whenever I didn't understand something, I would always go back and teach it to myself using the textbook. I found that my teachers often used different methods to the textbook, and if you don't understand their way of teaching you can just look at it in a different way. I have friends who did badly because when it came to revising they only did the topics that they didn't understand, whereas you should do entire past papers to get a real feel for it, and to spot your mistakes. Because I reviewed all of my mistakes using the revision cards, I found it easier to remember them when I did the exam, and I had several checks that I did on my answers. I came out with an A in further.