First things first, have plenty of your favorite beverage. This job can be very aggrevating.
Light weight nylon cord will work well. It is easier to install with the frame on the car, if that is a factor.
Install the gasket into the frame. Insert cord into inside groove of gasket. Dish soap helps to keep the cord in place and works well as a lubricant. Ends of the cord should exit bottom or top center. I prefer it on bottom.
Work glass into gasket at position where you have the cord exiting. As one person slowly pulls the cord ends, the other controls the glass, applying light pressure keeping it close to the gasket and in position. If you don't maintain this pressure, you won't get the glass in the gasket, and your cord will just pull out of place. Pulling the cord pulls the outer lip of the gasket out, allowing the glass to fit into place.
The person holding the glass is best positioned in the driver's compartment, while the puller stands outside the car.
You will find that pulling one side of the cord at a time is best most of the time.
Don't apply too much pressure for obvious reasons.
Work slowy and together.
If you listen to Lawrence Welk while driving in your first rain storm, you will get the authentic visual. Unless of course you rinse the soap very thoroughly when finished.
The plastic knife can be used to work the gasket edges into place.