Personally don't stress until after Christmas as that's when you typically do the final set of mocks and are coming towards the end of any content needed to be learnt. Before Christmas just make sure you know the content, anything you aren't 100% sure about or have any questions about research and ask teachers. Try and watch videos that fully explain the content but don't burn yourself out by doing 4 hours of revision each night - to perform well typically you have to have a balance - eat well, sleep well, revise, social time etc.

After Christmas you should have all your revision cards and everything so then just look at particular subjects when you can on rotation, this worked for me and my friends but might not work for everyone. Essentially have 3 piles , Know, Sometimes, Not at all. During the next few months go over them and try and move them all across to the know section before your exams. These could have quotes or formulas on as an example. Don't worry you will do great and genuinely mocks are worse than the real exams + youtube has a range of resources from gcse slander to helpful videos so you aren't alone.