Man people leave things this late before they start revising systematically, even for their full GCSEs (though I don't recommend it, or course).
There are many different ways to do it - you will find your own. However, from my own experience, I would keep in mind the following.
- The most important thing is being systematic and thorough. If you don't understand something (this applies mainly to maths and sciences), understand it before you move on. It is better to do most of the course thoroughly and skim the rest than to skim it all and leave big gaps.
- With maths, it's all about practice. Do every exercise in the book (every even question / every third question etc if you don't have time or have a massive textbook). Mark your own answers and work out why you were wrong. I can't stress this enough.
- With RS, you have probably absorbed more in class than you think. However, you should still go through the syllabus systematically, making sure you understand everything, and have a list of quotes to memorise just before the exam. If you have time, read around the subject - dropping in an off-syllabus philosopher could look very impressive at GCSE (but make sure you get the context right, and the syllabus is your priority).