I changed my'n not to long ago for a heated windscreen, was a pain in the arse until my dad came over and made me look like an idiot.
We warmed the seal in some hot water and placed the seal around the window whilst it was led flat. Then cut two lengths of string, put the string in the frame slot of the seal, the idea is that each piece of string goes from the top of the screen across, down and meeting back in the middle at the bottom making sure that top and bottom cross each other by about a foot.
Lube the seal and the frame up a bit then lift it into place, one person puts a bit of force on the windscreen and the other begins to pull the string, the string pulls the seal over and jobs a good'n.