You will need to create a template to cut your window hole. Easiest way is to use the cardboard box it came use a square and make a rectangle . Approx 29"L x 7-3/4" H always double check measurements.
Once you have your rectangle created line up your rectangle on to the inner framing on the bunk window you should now be able to hand draw your corners, if you have a protractor great if not by hand generally suffices. Once you have your corners drawn and cut out you should now have a perfect template for your bunk window. Always double check to insure you have everything correct.
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 but allow a 1/2" of gap when possible as when the urethane is applied it will spread and you do not want to urethane seeping out on either end as its very tedious to clean. Allow primer to dry for about 10 mins, now you can apply your urethane, most of the urethane will come with a v-notch or an etch for a v-notch in the tip this allows for a nice large bead of silicone make sure you evenly distribute to urethane to not have high or low points when changing urethane overlap the existing bead slightly to insure there will be no gaps.
When placing the Half-Slider Window you want to get it as close to desired location as possible one in place you will have some 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.