It's a pain in the arse!
Done a few now and everytime I say I'll never do another
You need to take all the windows out and the door seals off.
I used some aerosol spray tack to glue the edges up but it's messy so you need to be careful not to get it on the car bodywork/interior.
The liner tends to be rotten around the tailgate aperture so it's best removed from the front backwards, being extra careful at this area.
You'll find when you're refitting, you'll end up removing bits you've already glued up until it's tensioned correctly.
'Scuse the dirt... It's not been cleaned yet