Hello hello! First off, well done for being so diligent! Its really good that you're already conscious of this sorta stuff! So ,what I'm gonna tell you about when to start/how to start is my own experience so I hope thats okay, but everything works differently for different people! I actually ended up running revision for others in my year and the year below because of the amount I managed to learn.
However, compared to most people on TSR my GCSE grades weren't actually very impressive, but the main thing is that I ended up going up a lot of grades!
OrganisationIf you're feeling overwhelmed that completely understandable, there are so so many exams to do in GCSE that its hard to feel at ease, but thats alright, let me tell you what I did and maybe you'll get some ideas. Planning will defo help your feeling of being overwhelmed!
Okay so the day after I got my mock results back in December I went home (cried) and then spent until early January going through the syllabi for each of my subjects and making a comprehensive checklists of every single little subtopic for each topic for each exam for each subject. By the time I was finished it was something like 20 pages long! They looked a little like this (though Biology was my most simple one, its just a good illustration!
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I printed them off blank and scheduled them in pencil so I could change them easily (which I did!). I got myself a big wall calendar (the kind with the big squares) and wrote the topic I needed to study that day on the postit and stuck it to the calendar so they could be easily moved.
So what time scale are we thinking?December - January : Researching exactly what is on the exam and roughly timetabling it
January-February: Start making revision resources you can use later - now is your time to get creative and make great stuff for yourself
March- The exam period : Start using those resources to revise and review and do practice papers! Practice, Practice, Practice!
Best of luck! You can do it! It's hard but you will make it and this time next year it will be nothing but a cleared obstacle!