do many practice papers all the time, find the mark schemes and look at each step of their working, learn what each step does instead of memorising it - i had a really good maths teacher who would show us the proofs to all things we must learn which helped us to understand why we do it and in what circumstances it’s appropriate. it helps for the tricky problem solving questions where you’re not sure what you need to do immediately.
watch youtube people severely underestimate how good youtube is for understanding concepts and for walking through pastpapers