Pull the seat forward and tip it forward. They are at the back of the rails on the floor. Usually just a plastic cover that pops off. Mine was a bench, so there was nothing bolted to the side of the transmission tunnel (hump), and when I installed buckets, I just kept the seatbelts in their stock location. I suspect you'll have to remove that part too though.
You'll need a 13mm socket and a ratchet. The seatbelt is a Torx, probably a T-45.
Here's a pic of the underside of the passenger-side Mustang seat I swapped in using Ranger bucket tracks, so you'll be able to see where the holes are. The outer hole is father back, while the inner is farther forward.
Hope that helps.