I got an 8 in maths, and it is just non-stop practice. It helps to watch videos on youtube on topics that you are weak on, then to practice exam style questions on maths genie (that website has good videos too). If you make a mistake identify and understand it rather than just correcting it and moving on i.e. maybe you didn’t use the correct equation properly.
Afterwards definitely practice as many past papers as possible, ideally in timed conditions too get a feel of what higher GCSE is like and what you are working at. Bare in mind about 50% will get you a 6 and 60% a 7, so work towards that goal. The more practice you do, the more efficient you’ll be in maths.