A leaded glass window such as stained glass isn’t difficult to repair. Since a lot of detail is usually involved with leaded glass, it will take a lot of patience, and some experience in glass cutting and using a soldering gun is very helpful.
Remove Window - Removing the window from the frame is the best method to fix a leaded glass piece. This will allow you to lay the window flat and work up against it very closely. One the window has been removed, trace around the frame on to a piece of cardboard or plywood. This will give you a pattern to work with when you start repairing the glass.