For the new specification for Mathematics and Further Mathematics, all you have to do is read the exam board endorsed textbook, follow the examples in said textbook (if you still don't understand the concept, you have plenty of resources available - classmates; teachers; the internet...), and do all the questions in the textbook. Once you've done that, find more questions, especially exam-style questions or difficult questions that push your understanding. CGP has recently published their exam practice workbook for both of those subjects and PhysicsAndMathsTutor has questions banks.
And, the way you should do these questions is by creating a constant feedback loop.
Step one: attempt question.
Step two: look at the answer.
If you fail, do it again.
If you fail again, do it again.
And you keep doing it until you're sick and tired of it.
Step four: look at the worked solution.
Step five: if you failed to correctly answer the question beforehand, reattempt the question without looking at the worked solution. If you have correctly answered the question, readjust your solution to look more like their solution or redo it the way they have. Even if you have the correct answer, the worked solution may give you a quicker way to do it or one where it's easier for the examiner to give you marks.
Step six: repeat.
That's how I revise those subject and I've been coming out with as and bs when I give a **** (and I generally give a ****).
Have you noticed something? It's still rote-learning!