I think that by the seal Sean really means what you've said. You'll need a nylon or silk rope (something like a parachute cord) - it must me strong and slippery.It must be about 4 mm thick. Put a seal on the window and take a look at it's cab side. There's a channel as Sean called it - it's a place for the flange of the cab. Take a rope and put it into this channel all around the seal, starting at the middle of bottom. When done, you should have both ends of rope hanging down of the seal and they must be al least 1 foot long each. It's better to have them overlaying each other inside that channel for 10" length. Have someone put a window against it's position from the outside. Have both ends of rope inside the cab. Start installing the window from the buttom. Make sure that the bottom of seal is at it's position. Your helper should start pushing the window towards the cab. Start pulling out the rope by any of it's ends. Your helper should watch for the window not to move while he's pushing it. Pull the rope out slowly, try to keep it at 90° angle.
There are three ways to do it - good one, bad one and russian one