Step 1 Install the seal onto the glass
Step 2 Moulding on the seal
Step 3 Assembly in the car.
It is possible to install the moulding onto the seal first, but its like
juggling jelo.
When you actually install the assembly, a number of issues can pop up,
Is it a new glass, some fit better than others.
The same with the seal.
Are you using a windshield string?
Use a lubricant.
Start from the lower center and work out and get one lower corner then the other , then up, one corner at a time, then finally the upper center section.
DO NOT force anything, especially the corners, be prepared to do this a number of times before you get it installed.
I have found it helpfull to , before starting , soak the new? seal in very hot water , and some dish washing detergent for lube.
Good luck
Burky.