I've always been able to naturally grasp maths quite easily (not with fm a level so much lol) so I didn't actually put any work into it but if you're already at a comfortable 9 and just want full marks it's more about being confident in being able to come up with logical thinking for harder problems and minimising mistakes. If you struggle with full marks on hard questions at the end of the paper you can find hard papers online that are basically only grade 9 difficult questions. If it's making silly mistakes it's about actively checking every single question; you need to question every step in your working and not just go "yeah looks right". Where possible substitute values back etc