If you don't have your side panels on yet, things are a bit easier. The crotch belt and inside lap belt should be installed before the seats. The outside lap belt should be installed before the seat if your side panels are already attached. Unfortunately I don't have pictures of these attachments.
My preferred method is to tightly attach the brackets to the seats, and then put the mount bolts on a 2x4 on a floor jack under the car. Raise the floor jack while aligning the bolts so they're held firmly in place through the frame from the bottom of the car. I am consistently able to get only one bolt aligned, so loosely washer-and-nut that bolt. Lower the floor jack so you can get your hand on the second bolt and wiggle it until you can get it generally aligned with the bracket, then shove it back into place with the 2x4 and floor jack. If you can get the bolt into the hole in the bracket but can't get threads to go through, just put your weight in the seat.
I have the FX1 seats (not pro) with the brackets mounted in the outboard position, with the tabs facing the inside. The nut for the inside bolt can be hit with an impact with extensions. The outboard nut requires a wrench as a ratchet is too tall and would hit the bottom of the seat.