Here's my 2019 addition to this very helpful thread.
Clean the groove in the windshield frame. It cant be too clean.
Any lubricant which won't hurt the gasket is OK. Like Barbara Walters, I used Astroglide. Get it in the groove and only on the parts of the gasket that go in the groove. Like Sally used to tell me...." Too much lube is almost enough."
I got an inch and a half putty knife at the local IGA for $1.99. After dulling the blade, it worked perfectly to push the edge of the gasket into the groove.
Fit the two bottom + side corners on before doing the bottom edge. It wouldn't hurt to start in the very center of the bottom after getting the two corners in...if you can.
I used three small, orange handled spring clamps to hold the gasket loosely into place, so I didn't have to keep reaching down for the gasket while working it into the groove.
DO NOT stretch the gasket. Consciously make yourself only push the gasket down into the frame, and do not pull the gasket tight.
In the unlucky event you do stretch the gasket, all is not lost. You can heavily lube the gasket in the frame and work it corner to corner as needed. Glad I figured that one out!
Don't trim the two loose ends of the new gasket until the windshield is reinstalled. Its really easy to think you're cutting them perfectly, yet once reinstalled in the truck, you may have cut them a bit shorter than perfect.
Between 20 minutes and 90 minutes per side should be a reasonable time goal to set while installing these gaskets.
Hope this helps.
Jay