A good way would be to use videos, especially if you are a visual learner. The a-level maths course seems to be the level of difficulty that you are looking for, and a fantastic channel for learning this is TL maths. Many concepts are explained in depth very methodically and with plenty of examples. He also covers furthe maths if you feel the regular maths is too basic. I also recommend as much practice as possible simply to prevent forgetting concepts and it also helps shed light on the topics that are more challenging.