First tip. Unless the gasket you are going to use is in perfect condition, get a new one. Those things deteriorate over time and will cause tons of leaks and problems.
Second tip. Get some .125" diameter cord. Put the gasket around the window, then lay the cord in the outside trough of the gasket all the way around. Place the window/gasket/cord up to the hatch, placing the point where the cords cross over in first. Try to hold the window/gasket up to the hatch. From the other side, slowly pull the cord through. This will cause the lip of the gasket to pull along with the cord to the other side and thus seat the gasket on the hatch. I know that is a crappy description, but ask one of your buddies to show you. You can swap a window in 10 minutes or less.
Personally, I then take a plastic syringe with rtv and go around and inject a small bead of rtv under the lip of the rubber gasket to both seal it and help secure the glass.