For maths, my revision consisted of 3 steps:
1. Write out the basic process/techniques within each module with one example for each - e.g. I would do a set of notes for C1, with subtitles like 'algebra' and 'differentiation'. For each module (e.g. C1) this usually takes up 2 - 4 double sided pages. I'd just basically summarise my class notes. I could then refer to these whenever I got stuck on questions.
2. Make cards with formulas/things that don't come kind of instinctively and that you just need to learn off by heart. Each module usually needs just 1 or 2 revision cards. Practice these lots, rereading and repeating them to yourself or writing them out lots.
3. Practice questions and past papers! Do practice questions on tricky topics but otherwise just keep going with past papers, making sure to check answers and write and understand solutions.
I would begin these steps maybe about 6-8 weeks before the exam, ideally. For AS this was fine but A2 I started a bit later so was a bit more rushed, especially as I had chem and bio. Over the year, I didn't really worry, just kept up with classwork, writing notes in class and doing the class exercises and homeworks.
For chemistry and biology? Well..I don't really know, I'm not very good at those subjects and probably shouldn't have picked them for A level! I typed up notes using the textbook and class notes, but for biology that was like 90 A4 pages, and chemistry about 50 A4 pages. Then I hand wrote flashcards for each, condensing things but not removing any information. Biology I ended up with, for the whole A level, about 120 flashcards. Chemistry was only about 60 flashcards. Hand writing them definitely helped me remember them.
Then I did past questions. And kept on testing myself on the flashcards and then trying to scribble out the information or say it aloud without looking at the flashcards.
Hope this helped x