Here's my advice - I sat my maths GCSE early because the top sets did at my school, higher tier, and to start with I was really uncertain as to whether or not I could perfect everything in such a short time. My first past paper, I got a D. What I did was I then starred each of the questions I got wrong, found the mark scheme online and found how to do each of the types of questions I struggled with. I then memorised these techniques and repeated the process. By the end of it, I was getting A* in practically all the papers I did.
Good luck!