If you are replacing an existing factory window that glued in you may want to take to a professional for removal and replacement. Best tool to use is a wire cutter  Equalizer Window Cut-Out Set . Once the window is cut out you will need to cut the original urethane down as flat as possible and clean as much away as you can CRL Adhesive Cleaner  .

Once the Hole has been cut out or window removed its time to install the new window.

The CRL Window utilizes a trim ring (clamp ring) installation. Before putting the window on the van its best to go through make a small pilot hole in all the locations around the trim ring that way you do not round the heads of the screws when tightening the window.

Put Window in place from outside of Van allowing the aluminum mounting frame to extend through hole in wall. Place clamp ring from inside most vans will have the split on the bottom. With one person holding window in place on the outside the other can start a the screws from the inside generally start with the two at the split in the trim ring to have that lined up then top and around you do not want to fully tighten once all screws are in place, you can then go back around and tighten them be sure to tighten evenly around for proper seal.