First you have to choose a string. I like a string cotton string. If you can get your hands on the string from a set of horizontal blinds, use it. It even has a handy handle to pull on. If not, find something similar. I stay away from nylon strings because they can be sharp enough to cut the windshield rubber slightly.
Add soap. Now you have to soap up everything. You want your string, your rubber and the car body to be nice and soapy. You can use a spray bottle, sponge, rag, whatever. Use regular dish soap in water. You can't use too much soap.
Wrap the string. With your rubber installed on the windshield, take the soapy string and, starting with the top center, wrap it tightly into the channel that holds the car body. Wrap it all the way around until you're back at the top again. Leave 6 inches or so of string hanging out from both ends, these will be your pull handles.
Install the windshield. Press the windshield into place, using your hands to press the rubber as far into place as possible. You want the rubber to be right against the body, ready to pop into place. Now's where the string comes in. If you have a helper, have your helper put slight pressure on the outside top of the windshield. If not, no biggie. From the inside of the car, take one end of your string and pull it gently through until you see the rubber popping through to your side. Gently coax the rubber through, and as you pull the string the rubber will follow. Move all the way around the windshield and by the time you get to the top again, it's in!