This is my advice to all current and future year 12 who are taking alevels. Start from day one.
For a-levels you must make sure you keep on top of all your work from the beginning. A-levels has a lot more content than GCSE and the speed you go through it is quicker than GCSE so it's better to be on top of your work at all times.
Being consistent during A-levels is very important, working for a few hours every day is much better and effective than cramming for a short period of time. It will prevent you from burning out and ensures you will see the results you want.
When doing past papers focus on the mark scheme and learn it if you get a question wrong. Especial for A-level sciences the mark scheme is very particular and you must write certain words/ terms.
Be organised, don't leave your work till last minute as it will add unnecessary stress for you. For A-levels you should try to reduce the stress you put on yourself as it is a stressful period as it is.