I've just recently done Harold, a GT. As I recall there were four snap studs in each footwell, and four along the rear seat shelf lip.
The carpet pieces under the front seats don't need securing at all: the seat bolts hold it in place nicely. I glued the sill carpet pieces with 3M trim adhesive: worked well.
The main challenge for the snap-ons was with the thickness of the footwell carpets that included underlay: the prongs on the male piece weren't long enough to poke through. I ended up just putting them through the underlay, which is stuck to the carpet above anyway. Once the prongs are pushed through, inserting the ring with holes over the prongs and bending the prongs over is easy, with a pair of pliers.