for subjects which you can't really cram for, i.e., maths, languages etc..., practice little and often. For other subjects, doing past papers and collecting questions which you have got wrong, making your own notes (outside of school) is what I would advise. If you don't get a concept, you could always ask ChatGPT - it's surprisingly good at explaining concepts in a comprehensive way - or watch youtube videos.
In the actual exam, make sure your writing is neat and legible. Examiners tend to mark more harshly if your writing is hard to read