Best method to revise for a gcse maths exam?

I did terrible on my mocks, got a high U (about 15%), so I need to drastically revise, anyone have any methods for this? I’m a decent student, normally get around 75% in my subjects and get As. I’m not so sure on what to do this time for maths, as I’m a hard worker and so on, but I don’t seem to get it. Any advice would be great :)