start the lip of the rubber over the steel flange by hand, or gently with a screwdriver. With someone keeping pressure on the outside pull one of the cords from the inside to continue the lip being pulled over the flange. The person on the outside should thump the glass too to force the rubber home. If it's the factory toughened glass it'll take quite a bit of thumping without any problems.
this is the technique that works, just keep at it