What's the procedure for removing the rear seat in a 1968 convertible?

Push back each bottom front corner about one inch and lift up.
Lift up on the front edge of the seat bottom and it will 'un-roll' from the bottom of the rear cushion.
Unscrew two lag screws from the floor.
Lift up on the rear cushion to remove it from the hooks.

If you don't have the front seats out, you may want to do that to give you some room. I had to get in a four point stance to force my rear seat bottom back enough the get it out of the anchor. HUT HUT!!!

There is so much information available here at TC, just waiting for the search.

If you search "remove rear seat" there are tons of threads, including some with pics of the hooks on the floor, so you can better understand what you need to do. You do push back and lift up on the bottom, but the hook has a lip on it, so you may actually have to push down as you push back. Once you search and see the pics, it should make sense.