Note the relative position of your old glass before you remove it.
Don't forget to drain the oil. The sight glass is literally the only thing holding the oil in.
Take a small screwdriver and carefully remove all the remaining bits of glass.
Using the same screwdriver, remove the metal ring that held the old glass in.
Remove glass from freezer, completely coat it with fresh oil, and VERY CAREFULLY, drive it into the case, using a small socket to spread the force of the impact(s) onto the metal rim of the glass.
Caveats
Be VERY careful, otherwise the new glass will crack, too. Very soft, light taps should get it in there.
Don't push too hard on the clutch cover; it's cast aluminium.
It might not be a bad idea to have a piece of pine or other soft wood around for impact absorption. (ie... to tap on)
If this is done during cold weather, you should consider leaving the bike overnight in a heated garage before trying to install. This, along with freezing the new glass, will help make the fit less stubborn. It's a simple compression fit; there are no bolts holding it on. Warming up the engine in the garage will help expand the metal and make the fit easier.
