Set aside one or two days per week to revise science, and do one topic on each day (spend about one or two hours).
Make notes from a textbook (CGP or other), or use YouTube videos to make notes (GetToKnowScience is a good new channel I've found - search it in YouTube) and then try some past paper questions.
Then mark it using the mark-scheme, and make corrections. At this point, if you don't understand anything you can ask a teacher or a tutor or whoever can help.
Then repeat this procedure for every topic. If you do a couple of topics per week you should be in good shape for the exams in the summer.