Hi, For CONTENT-HEAVY SUBJECTS (Science, Geography, Computer Science, RS) don't worry what grade you are on now, you can get a 9 easy by GCSE in 4 months. The ONLY thing you need to do is find out what exam board your school is using for each subject (ask each teacher, it is usually one of: AQA, OCR & EDEXCEL). Now find the exam boards SPECIFICATION for that subject. This is a PIVOTAL document that lists out EVERY thing that you need to know for the exam (not a dot more, not a carrot less). Now you just need to ensure that you KNOW everything on that document (some things you might know from school, but if you don't know it then find videos about it).
When people talk about 'revision notes' the MOST EFFECTIVE (basically guaranteed 9) notes are when you use the SPECIFICATION directly and make notes for every single point. Then just read through the notes constantly and you will get a 9. The problem is making these specification based notes is long, boring & people don't know where to find the resources for it.
To tackle this problem, I would highly recommend checking out notes on stuvia (a website with revision notes) made by 'CSandRS'. Just type CSandRS into the stuvia search bar. I personally used those notes and got a 9. They make notes in a unique way by putting the exact specification point in the document, then nicely explaining it. So you just need to read the full thing and you will be sure that that is everything that could come up on the exam. Just make sure you use the same exam board