The first thing you need to know is that they now make a totally closed molded rubber for the windshield "Maybe even for the back window now?" to fit the opening so now you don't have to worry about a gap where the two ends meet in the opening. This makes for a cleaner installation!
To begin the installation place the glass in the rubber molding. If your using the molding with an open end then place the molding on the glass so the two ends will meet at the bottom center of the opening not only for aesthetics but for less chance of leakage.
I then take a thin cord string and cut a piece that will wrap all the way around the inner grove of the molding "where the molding will install on the lip of the cab". Have the string overlap itself by a foot on each side. Place the cord into the grove of the molding and have it overlap in the center at the top.
Mix up some lubricant to aid in the installation of the molding in the cab opening. I take about " 2 or 3 squirts" of dish soap into a small squirt bottle and fill the rest with water. I squirt a liberal amount into the inner grove of the molding where the string is installed and the lip of the cab where the molding is going to be installed.
Now I have my Helper "MANDATORY" raise the glass assembly up to the window opening and position it close to the lip with a slight pressure on the area where I intend to start pulling the string. As I pull the string from the inside at a downward angle, and with my helper applying a light pressure on the glass at the same area from the outside, the downward pull on the string will force the rubber molding to pull inward and wrap around the cab lip. I follow this procedure all the way around the glass. The key is to have a helper that maintains some pressure at the starting point so the rubber molding stays on the lip and the other hand follows the string to help the molding pull over the lip.
Once the molding is installed it's time to install the locking bead strip. This will require a special installation tool that you can pick one up at any Glass Shop or order from a vendor. I've seen them in the Chevy Duty catalog for $10 and their probably available in others as well. What the tool does is expand the molding opening and feed in the locking strip at the same time.
Some things that helped me install the locking strip was to: