Forney rear window installing

I know there’s a system for installing a rear window on a Forney. What is the trick or procedure for getting this done W/O breaking the replacement window? I thought of heating the gasket first; starting from the big side, or small one; putting gasket on first, or leaving it in and working the window in. Many approaches, but others have discovered the keys to success. Please share your experience...it will help a bunch. Thanks......Gale York