THIS IS HOW I DID MATHS:
For maths use the website onMaths.
Go to the topics section and work through all the ones you struggle with, all the higher ones if possible.
The first time you do a topic test its OK if it takes long. Use other websites to help you learnt the topic so you can answer the question. If you get a question wrong refresh the page and it will load the same question but with different numbers so you can try again. Theres usually only a few questions in a topic test, but they get progressively harder so it doesn't get boring and ensures you cover the whole topic quickly.
Once you've gone through it once and understood how to answer all the questions do it again and again until you don't get anything wrong(if you drop one or two marks for silly mistakes it's OK) and dont need to refresh . Try and finish it faster too (if you take too long repeat it, if you get something wrong finish the rest of the questions in the tests then repeat it.
Personally I think the most important skill is to be able to factorise quadratic even when coefficient is of x^2 is not one. So if you cant do this alresdy, then start with it.
Use spaced repition: once youve got it right do it again in about 2 days, then in about a week(if you didn't struggle) then in about two weeks. Any ones that you struggle a lot with repeat them more often. Each time you start a revision session do any ones you've done before that you need to do again and then try a new topic. Eventually you will get most/all the topics in higher done.
This will take a month or two.
Then, by December at least, if you've finished, just do past papers. You should find that most of the questions you can do without even thinking about it coz you will have done them lots of times. There will be a few questions where you have to think a bit more to decide what technique you need to use so you can spend more time on those.
You should end up with an 8, at least, with this method (me an my sis did this method, she was working at a 3 at the end of yr 10 and I was working at a 6 when I started this method but I did it halfway through year 11)