How long before GCSEs should I start this method?

As soon as the course has enough content to diagnose. Year 10 is not too early for a snapshot. Year 11 spring is late for a first diagnosis but not pointless.

Does watching videos count as revising Maths?

Only if a marked question follows. Watching someone else factorise is not factorising.

What if I get stuck on every Higher question?

Check the tier with the school. Revising the wrong tier with this method still wastes time. Then diagnose on the correct tier.

How to revise GCSE Maths — a method students actually follow

How to revise GCSE Maths without pretending you love past papersHow to revise GCSE Maths step by step: diagnose, drill one skill, mix, sit timed, mark honestly. Includes Foundation/Higher notes and a free test.Start free GCSE Maths test · Pricing