Normally when you buy carpet the underlayment is already attached to the back side of the carpet. You have to cut small holes in the carpet & underlayment for the seat studs. You will go through a few razor blades. It dulls them pretty fast.Real nice Jr.
While I got you, what's under your carpet? Are the seats bolted through the underlayment with the underlayment sandwiched in between?
I'll be installing carpet and seats in the near future and don't really have anything to go by. The carpet was missing when purchased.