You will need to create a template to cut your window hole. Easiest way is to use the cardboard box it came with lay bunk window on it inside up.
Measure out to create a template from the see-thru glass area of the window
When drawing template to Van if there are two people great one can hold while the other draws it out. If your installing solo once you know the spot you will be placing the window you can screw the cardboard into the side wall to hold in place then draw your pastern.
Cut out your hole with your choice of tool reciprocating saw, sheers,disc, etc..
Once the Hole has been cut out its time to install the Bunk window clean the outside metal of the van with rubbing alcohol or PWR22 Cleaner this will insure a good hold of the urethane eliminating contaminants.
Using a urethane primer apply to the outside of the body you want to be close to the opening tape off the wall 1/2" away from the opening and primer within. Allow primer to dry for about 10 mins, you will cut your urethane tip straight across no V-notch you want it to be a tootsie roll size of urethane make sure you evenly distribute straight to the fret of the window itself not onto the wall.
When placing the window you want to get it as close to desired location as possible one in place you will only a small amount of wiggle room to get it lined up just perfect once you have it where you want with an open hand pat all the way around the window where the urethane bead is to get a good solid hold. Using Window securing tape , to hold the window in place. You will be able to drive your Van after 3 hrs and remove the tape after 24hrs, best to wait 72hrs before washing vehicle.
There will be urethane then oozes into the interior of the van, using glass cleaner spray the urethane with a liberal amount then using a plastic putty knif smooth the urethane out like you spackling a wall to insure complete coverage of the hole and a nicer appearance.