What I've found works is to get the gasket out in the sun (in the UK?) so it's soft and flexible.
Install the gasket around the glass and then set the bottom edge of the gasket over the flange at the bottom from the outside.
At this point, put in a section of twine tucked into the outside groove of the gasket along the sides and top. Often some soapy water or glass cleaner will help get everything slippery.
Have a helper push gently on the outside of the glass near the outer bottom corner, spreading the area they are pushing on. While the glass is being pushed in, go inside and pull the twine so the lip of the gasket flexes up and over the flange.
You can push pretty hard before the glass breaks, but pushing also tends to jam the gasket edge so it won't slip up and over. Work slowly, and work up the to proper force.
Once the gasket is up each side, work the top corners. It may require shoving the glass down into the base gasket to get around the top corners and the top of the gasket to slip in. A tool like a bent screwdriver may help coax the gasket to flip up over the flange, but be careful around the glass edge.
After the gasket is fully installed, grab the glass between your hands (in and out) and wiggle it so it's centered and the gasket is even all around.
Now install the lock strip in the front of the gasket. There is a special tool for this, but if both the gasket and lock strip are new, warm and flexible, it can be done by hand with a couple screwdrivers or plastic trim removal tools. Be careful around glass!
You can google images for a better view of the
windshield lock strip tool.
BTW, will the guy cutting the glass install it?
If so, he takes the risks, has the experience and should warranty against leaks.
If the glass is slightly off, he will have the feel to know and fix it, too.
Here's a youtube video of the process
https://www.youtube.com/watch?v=keNGeBKOduMHere's one installed the opposite way, gasket first, then glass. I have never seen this done in the field.
https://www.youtube.com/watch?v=mPBZOVK0eBI