From doing this myself, I found you need three items then its a fairly easy thing to master.
1. A suction clamp - I got one of these which is brill
http://www.screwfix.com/prods/18817...ter?cm_re=SEARCHPROMO-_-GLASS SUCTION-_-18817
2. Some silicone spray lubricant like this
http://www.screwfix.com/prods/13662/Plumbing/Plumbing-Accessories/Silicone-Spray-400ml
3. A strong piece of thin cord that you can wrap around the inside of the rubber seal and overlap by at least 6"
Fit the glass into the seal*, wrap the cord around (inside the lip of the seal) and overlap it. Spray silicone over the cord/seal. From inside the van with the clamp on the glass, pull the glass firmly into the window frame whilst pulling the cord and therefore the lip of the seal over the metal frame - bingo! Might take a bit of practice, but its easy to get the hang of.
*This is for factory fitted windows - if you have a panel van conversion (ie window frames are simply holes cut in the side of the body panels), fit the seal to the frame first, cord inside the seal lip, push glass and pull cord from outside, then fit the filler strip round the window seal (there is a special diamond shaped wire tool which makes this easy to put in. I know some people who have put the filler strip in before the glass, but I imagine this would make life much more difficult to get the glass in.