Knowing how to revise is one of the most important skills that every student needs, but it is a skill that we are often not taught.
Don't fall into the trap of assuming that all you need to do is read over your notes and do a few past papers! Some people may be able to get away with this approach, but for most it may not produce the results they had hoped for.
So, how does one revise? Mindmaps? Past papers? Making notes? Flash cards? Mnemonics? The honest answer is: it depends. But the principles are always the same.
Stage 1: Do you understand the main concepts?
At this stage you could read and make notes on the topic, or perhaps constructing a mindmap, flashcards, or something similar might help. It depends on how you like to learn.
Stage 2: Can you apply what you have understood?
This is when you could try some past paper questions, or some questions from the textbook.
Stage 3: Feedback.
So, you've learned your topic and tested yourself. Did you get full marks? If not, you need to figure out why. At this stage you could ask a teacher or tutor for help, or you could sit down with your textbook and try to figure it out. Did you simply misapply an equation, misunderstand a concept, or do you need some guidance on how to answer exam questions? Asking someone with experience can help a lot.
As you may have guessed, this takes you back to Stage 1, hence why I call it the Feedback stage. You now need to go through each stage again, until you get the marks you want.
When should you do all this? After Easter? NO! This method works best when it is applied consistently, every week, all year round. You need to allocate some time to your science revision each week - an hour will do. Perhaps at the end of each week you could revise the topic you learned in school that week. Or you could do it ahead of time, to give you an advantage over your classmates. Whatever you choose, be consistent.