I recently removed the rear seat on my E30 for reasons I’m too embarrassed to explain here.
Months later, today, I put the seats back. There are some subtleties. For the seat back, I had to remember to move the seat belts so that they’re not trapped underneath the seat back.
At the extreme outside bottom of the seat back (the vertical part) there is a steel band that protrudes downwards, with a bolt hole in it. There is a corresponding hole in the sheet metal of the car, and the vinyl of the side paneling. It was hard for me to align the holes. I used a small screwdriver to explore where the holes are, and to help move things around until the holes aligned well enough for me to shove the 10mm sheet metal bolt into place and snug it up.
Before I put the seat bottom in, I got the felt cover in place over the sheet metal. I then snapped the seat belt anchors into their slots at the bottom of the seat back. Then, I moved the seat bottom into position. There are two tabs onto which a spring-loaded section snaps into position. The trick is to push the seat back and down, while grabbing it close to the location where the tabs are.
I just compared my 2-door 1984 BMW 318i to my 4-door 1985 BMW 325, and they seem identical as to how the rear seat re-installation works.