In the workshop manual under "body" Pretty vague though has to be said.
I did the replacement on my old plus 2 and was time consuming but easy enough, except.
1) Remember all the screws along the bottom holding the body over the inner seal are 2BA.
2) all the nuts and bolts are going to be rusted up.
3) It will pay you to order new seatbelt mounting plates whilst you are at it.
4) try not to rip the carpet when exposing the 2BA nuts and bolts, as you will have to glue the carpet back with Evo stik (and make sure that you do this in a well ventilated place, outside preferably).
5) Be brave when you pull the body open to get the new ones in, you will think that the body will not take it, but you will be surprised.
6) Be prepared to make up new wheel arch cover plates, if they are the original steel ones then they will have certainly rusted by now. I made new ones from Ally using the old ones as templates, and slapped loads of underseal over the top.
7) All the usual suspects supply the sills, CN Spyder etc. Perhaps best to go for the galvanised units, and slap loads of grease, paint or under seal on them to give them a bit more life when you replace them.
Regards
Jeff 72 +2