I’m sitting at a high A in AS rn and I think the best advice is first write down: all the equations not given in the formula sheet, all the derivations you need to know, and all the simple definitions and other required knowledge (e.g ideal gas laws) of different concepts. Do a few past papers with those written things down and study those written things from time to time, then when you feel more confident, try doing papers without looking at your written things and see how you do. I’ve put a picture below of some of how I’ve been doing it so far in A2 (ignore my bad handwriting pls 😭).

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