Just a tip fella..
Don't drill the window aperture in your bus with the window in situ.. you'll knacker the window frame where your screws have to go..
This is how I did it..
Mark out 5 locations top and bottom, 2 on each side, drill a 4mm hole with a good edge distance.
Then put the window in and, using short 4mm self tapping screws, fix the window in place. The screws go into the channel that has sawteeth in it.... make sure your screws are not too long as they won't clamp up nice and tight..
I've got very old seals on mine at the moment, so it took me approx 30 mins to fit both sides.. I'm looking forward to the fun and games described by others when my new seals arrive!