For memory, the back section of the back seat hooks onto lugs at the top, then you fold down 2 tabs at the bottom to hold the bottom of the back section in place.
The base section of the seat pushes into lugs at the back then pushes down at the front to hold it in place, I think ??
and FWIW they are a prick of a thing at times so you may have to play with them a bit to secure them, and yes, I do remember sitting on them can cause them to come unhooked, so you may have to use some clips/pipe saddles screwed over the seat frame to the floor/panel area to hold it, but make sure you are not screwing into fuel tank/pipes/wires etc.