Using small nylon twine, insert the twine inside the rubber gasket channel twice, with the gasket around the glass. Start at the top and put the twine in the channel and wrap it around to the bottom and then back up to the top. Lay the glass and the gasket onto the windshield frame and, using a helper who presses the glass and gasket against the lip, draw the small nylon twine from both ends. The rubber will flop over the lip and the gasket will be seated. Reinsert the trim pieces.

Fairly easy actually.