there are always tricks thats what separates the men from the boys. lol
are you saving the old gasket or going with a new one?
if you are just installing the glass than you leave the gasket (rubber seal ) in place.If you look close at the gasket there is a fine line in the center that splits. this is called a self locking rubber. I use a hook tool (cotter pin remover) and open up the seal all the way around then use the hook between the rubber and the glass to loosen any sealer (ureathane) make sure that its completely free then go to inside and place both feet in a top corner (its usually easier in the top right because of the steering wheel) apply pressure with both feet (this spreads the force over a larger area and allows more pressure without breaking the glass) WEAR SAFETY GLASSES HERE!!when the glass just clears the rubber move your feet towards the center and with less pressure keep it going across the top then down the sides. sometimes it helps to spray the rubber on the outside with soapy water or glass cleaner before you start with the feet.
after the glass is out clean as much of the old sealer as possible then soap up the rubber and slide the glass in to the bottom channel and get it as close to centered (with the gasket lubed you'll be able to move a little from side to side. the glass will be in the bottom and laying on top of the gasket on the sides and top. place your hook tool between the rubber and glass AND THIS IS IMPORTANT have the hook tool pointed in the direction that you plan to move get under that rubber lip at least 1/2 inch pull the rubber lip AWAY FROM THE GLASS don't twist the tool or use the glass as a fulcrum point.go about 10-12 inches up one side then move to the other side and go up around the top corner then back to the first side and around top corner and across the top keep it lubed with the soapy water. after its in, dry the gasket and apply the sealer between the rubber and glass. then close up the lockstrip that you started with then clean up the big mess you made when you put in way to much sealer.
note to close the selflock lube it again and with the hook tool again stick it in the groove and then lean it over the flap and slide it up or down and keep your index finger of your free hand behind the tool to keep it from popping back out again
now you know as much as I do and I've been doing this for nearly 40 years.
now if you need a new gasket I'd tell you grab the wallet and pay a pro. new gaskets can be a real ***** on chevy vans
is that enough tricks for you?? good luck its not that hard