Assuming your old guide is broken: .
Acquire replacement window guides. .
Carefully and slowly wind down the window several inches. The window will pitch forward and down.… .
A hole in the glass will become visible at the rear of the window. .
Wind the window down so that you have enough room to work at the rear of the window. .
Fold the new guide around the glass so that the new guide's male part on one side lines up with the hole and the new guide's female part lines up with the hole and press them together. .
They are usually very hard to press together, so you may wish to use vice grips. Do not press too hard otherwise you will shatter the window glass. .
Once the guide is secure, wind the window up slowly and carfully so that it presses the guide into the channel. ( Full Answer )