Last year I got A*s apart from 2 As in physics and Ad Maths. Honestly I was surprised at how well I had done.
The best way I found was to start making notes for fact based subjects at least a couple of months in advance so they're finished by study leave. if you're sitting your exams now then it's too late, just use any notes you do have and make sure you look at the textbook for reference. As for things like maths and physics and languages, just make sure oh understand them well throughout the year, and do lots of practice questions.
During study leave do as many past papers as possible and it's better to get someone to mark them for you as they are more likely to stick to the mark scheme exactly. This means you'll get a more realistic view of your progress.
Make sure you do the past papers so you know the format of the exam and any model answers.
In many subjects at GCSE there are model answers for certain 3/4 mark questions, you'll know what these questions are based on the past papers. They'll come up A LOT
And lastly as for the actual studying, just make sure you can grasp the outline of all your subjects then study them in detail. Whatever you do of you're aiming for all A*s don't neglect any subjects, not matter how easy you find them!
Good luck!!!