In terms of advice I can offer:
Chemistry:
Practice, practice, practice! You can know the content like the back of your hand, but knowing how to apply it to an exam question is what really counts. You can get away with using both old and new spec past papers, but the old spec papers tend to have less topic overlap and a few topics from them have been removed entirely (or moved from one year of the A level course to the other), so have a copy of the (new) specification handy if using the older papers.
Chemistry is extremely repetitive at A level and there is a lot of memorisation to be done. As such, active recall methods of revision will be essential. You can find all sorts of quizzes on sporcle.com for reagents and conditions for organic reactions, colours of ions etc you need to memorise, for example.
Maths:
The first paragraph from the chemistry section is largely applicable, just don’t bother using the old spec papers for stats and mechanics (unless you happen to do either in FM) as they are just a complete mess and there is so much stuff in them that has been moved to FM.
Further maths:
Same as the above really. Just don’t be too daunted by FM. It is fascinating and the grade boundaries are set so that somewhere between 50% and 60% of candidates get at least a grade A. Enjoy it and get regular practice with it.
More generally:
If you are going to make notes and flashcards, start at the earliest possible opportunity and do not leave it until year 13 only to panic at the very end.
Have a routine and make sure you get adequate sleep.
Study sensibly- don’t overwork yourself by continuously studying for hours on end. Take 15 minute breaks every 45 - 60 minutes as productivity usually drops off after continuous study for periods of that sort of length. It also makes dividing your time between your subjects easier and so you don’t end up overcommitting to one and forgetting the others.