First, go through the official exam textbook and make general notes on each distinct topic / paragraph in the textbook, and answer any questions in the textbook.
Next, do all the past papers you can under exam conditions and mark your work.
I'm sure you've heard that there is no one-size-fits-all method to revise; yet I would recommend past papers and mark schemes primarily. They're great because you can check where you've gone wrong yourself and learn what exam techniques to apply to that subject and others. Just pop this into the search box: '[subject name], [exam board] (OCR/AQA/Edexcel/WJEC/..), past papers' and they should come right up. Besides that, you should always read the textbooks regularly; if you have to make notes then definitely do that but if you have the sorta memory that can absorb information just by looking at it then that is good also. Reading the content regularly and taking books everywhere you go is a surefire way to actively know your stuff - that way you can just open up the book and have a quick scan in a cafe or on a train eg and often if you're using fun books like CGP there are lame jokes in there that make it a bit more entertaining more often than not, more information actually goes in than you might think. Just stick at it!! Good luck
I'm guessing your no from the uk... GCSE is a general certificate of secondary education so the exams you take to "graduate" from secondary school at around age 16 years old in the uk
Ahaha oh right aha I tend to be one of those people who read without having to read the full word so as soon as I saw G and E with an CS in the middle I thought it was GCSEs ahah