LASORS is the font of all knowledge on licence's. But as I'm in the mood to explain,
To go from a PPL to Instructor, you need to ask yourself one question first. Do you want to get paid to instruct?
If the answer is yes, then
1) You need to cart yourself off the Gatwick and get a Class One medical
2) You need to complete at least the CPL ground school and pass all the lovely exams.
3) You need to build up 150 hours, then you can start a CPL course.
4) You need to pass said CPL course, and before you get your licence issued you need. 200 hours Total, 100 hours PIC, Night qualification (or the training completed), a 300 mile cross country including 2 full stops at different aerodromes, and 10 hours on instruments.
5) You start and complete an instructors course.
6) You pay the CAA and they give you a shiny blue book with an FI(R) rating.
Total cost will be in the region of 12 grand.
If the answer is no, then
1) You need to complete at least the CPL ground school and pass all the exams.
2) You need 200 hours total, of which 150 must be PIC. You will also need a 300 mile cross country with 2 full stop landings, 10 hours instruments.
3) You start and complete an instructors course.
4) You pay the CAA and they give you back your brown book with an FI(R) rating.
5) You then start working for free, further reducing the existing terms and conditions of some poor instructor eeking out a living on baked beans.
Total cost will be in the region of 8 grand.
Helps?