For Chemistry, I've been learning the content using the notes on
https://chemrevise.org/ and if I have any doubts watch the videos on
https://www.youtube.com/user/MrERintoul. Then I do as many practice questions as possible using
past papers (use questions from different exam boards as well because the questions are similar),
questions by topic on Physics&MathsTutor,
http://www.a-levelchemistry.co.uk/, and
exampro questions that my teacher gives me (ask your teacher for extra questions once you've done all of the above).
Be aware of the practical questions which will be coming up in the new spec exams. I've been advised by my teachers to use the past ISA questions to practice for these.
For Biology, I use the official endorsed textbook to make short, but detailed, notes which I memorise. I then hit the
practice papers, which is very important in Biology. When my only exam prep was memorising my notes, I would get a C grade, but after I did a billion practice questions, I am now working at an A/A* grade. I do questions from AQA, Edexcel and OCR. The sites I get my questions from are Physics&MathsTutor, MathsMadeEasy, pxsBiology and SaveMyExams.
Maths is my favourite subject because if you understand the topics thoroughly, then you won't have to memorise much and can score highly in exams. I read through my textbook and try to understand each topic. Then I use examsolutions.net to confirm my understanding of the topic before doing a lot of practice questions. For the questions I use Physics&MathsTutor, MathsMadeEasy, CrashMaths and MadAsMaths.