If its an Isofix one, you just clip the seat into the Isofix mounting points on the rear seats, then strap the kid in using the seatbelt as normal.
My daughter sits in an Isofix child seat in the back of my GT - its a bit of a faff getting it in there (and plugging in the seatbelt) but once its in there its fine.
If its a non-Isofix seat, then I assume you will just need to follow the instructions on the seat to secure it via the seatbelt, then install the kid in the usual way.
I gave up with the idea. I opened the front door to it pissing down with rain and there was no way I was getting soaked trying to fit, so I will take him next time lol