I can tell you what to do for Chemistry since that's the only subject we have in common but I guess the general advice applies to all subjects.
Learn all the content first.
Do as many past papers as possible.
Have a document where you write down all the trends in the mark scheme, look out for all underlined words, look for patterns and really familiarise yourself with the content and papers.
I'm not sure what works best for you but use revision techniques that suit you best. For example, I do Chemistry and there are different types of questions they can ask you (definitions, trends in groups or periods with reason, equations). So I made a document where I wrote down ALL possible terms they can ask you to define and memorised them.
I separated organic and inorganic chemistry and wrote down all possible trends in one place with reasons. For example, "as alkyl chain increases, acidity decreases" then write down all the possible explanations for this trend, that way you can have access to a lot of 3-4 mark questions.
For numerical questions, make sure you are able to solve for all the variables in an equation, specially when they are not in their SI Units.
Going through mark schemes is very important. I would also recommend watching someone do a past paper on YouTube so you can understand the thought process behind every question.
Write down all important reactions, their reagents, their products, conditions and catalysts used and learn them. Also, know the reasons for why those conditions are used.
Categorise the content, break it down into smaller chunks and make it accessible.