i am the self-teaching master (been doing it from my GCSEs all the way up to my masters degree).
maths is all about past papers. don't go through your textbook or any of that sh*t. you start off by going on the examsolutions website and watch him go through each question (do this for say, 3 papers). after that attempt the papers on your own and refer back to examsolutions when your stuck (if the mark scheme doesn't make sense). maths (especially at A-level) is just the same questions getting repeated over and over again with different figures.
i got 91% in C2 if i recall correctly. never went to classes, never picked up a textbook, just went through the papers. looking back at it now there's no excuse to not 100% the exam. good luck brother.