For window installation I have followed the following steps with good results:
1. Install on a warm sunny day if possible, or in a heated garage (warm seals
stretch better and are more pliable)
2.  Place the seal around the window.
3.  Insert a thin but strong cord around the inside channel/grove (which fits
around the sheet metal lip once installed).  It is a good idea to have this
cord start and end at the bottom center and overlap at least a few inches.
4.  Use some type of lubricant in the grove (some people use soapy water, I
like to use surfboard wax, others use WD 40 or similar).
5.  Get your buddy to hold the window from the outside of the van firmly up
to the place it will go.
6.  From inside the van begin pulling the cord (which should cause the seal
to pucker out over the flange in the sheetmetal)
7.  Have your buddy follow your pulling of the string with pressure to that
spot from outside as you pull around the perimeter from inside.
Good Luck!
Loretta
87 Syncro
90 GL
82 Westy