I think it goes like this, you must,
1) Get an IMC rating
2) Build Hours to around 200 TT inc about 100P1
3) Study for and pass written exams to at least CPL level.
4)Do a commercial GFT course and aquire (at the moment) a BCPL.
5) Do a FI course
* If youre not interested in being paid then you can skip step 4.
Being a gormless instructor myself I cant vouch 100% that this is wholly accurate as things seem to change on a weekly basis. My advice is find your nearest training organisation who provide commercial GFT and Flying Instructor courses and ask them the same question. Most of us in the business of training PPLs dont necessarily keep up on developments as much as we should.
Good luck, MAYDAY