They are quite tricky, they have whats called inthe trade, a double lipped rubber, you have to get one over the body, the next over the inner trim, use a small nylon rope, or washing line, get a silicone tube nozzle, cut the end off so it'll go over the rope, then tie knots in both ends so it doesnt come off, use the nozzle to feed the line into the gap, start from the centre bottom and run it around the rubber...
When you 'string it in' do the bottom first, then one side, then the next, and finish in the centre at the top..
Use a hose removal tool (it's like a pointy screwdriver, that has a 90 degree bend in it) to pull the second lip over the trim....
It's easier to do than explain !
(I used to run a Windscreen Co.)