To begin with you should tape together what is left of the broken piece of glass. If necessary make a rubbing of the window. Tape a piece of white paper on one side of the window, lightly rub the window with a crayon or charcoal so that you can see the shadow of the lead came lines.
Next, remove the frame. If you have a zinc came frame, you need to use a temperature controlled iron so that you can turn down the heat. Then with the temp turned down, heat up and actually carefully move the solder from the soldered areas on the zinc came frame, use paste flux on the joint so that the solder will flow. This will allow you to "break" or open up the soldered joints. You'll have to do this on the corners of the zinc frame and the areas where the lead came is soldered to the zinc frame as well, remember to do both sides of your window. Carefully remove the zinc frame from the closest side to the broken piece of glass, you might have to remove two sides of zinc came.
Once removed, use your lead knife and cut into the closest soldered joints so that you will be able to open up the window to remove the broken piece of glass. You will have to cut both sides of the window, in the same spots. You might only have to cut up one area, but if the piece is large and you can't get into the broken piece, you will probably have to cut up into the window in two areas.
When you have cut both sides of lead came (only on the solder joints), front and back, up to the broken piece of glass, carefully remove or open up the window to where the broken piece is located. Once there carefully remove the broken piece of glass with needle nose pliers. Since the piece is cemented in place, it might take some work to pull the piece out. Use your lead knife to clean out the lead came surrounding the once broken piece of glass. If the lead came is damaged, you will then need to replace it.
Use the old piece of glass as a template to make a new paper pattern to replace the glass. Place the broken piece of glass on a piece of paper, trace around it with a pencil, and cut it out with regular scissors. Glue that piece of paper to the new replacement glass, cut, grind and its ready to replace.
If the piece of glass is too broken, or non existent, to use as a template to make the new piece of glass, use the rubbing you made of the original window to make the new paper pattern piece. We like to make a copy of that template first before doing anything else, just in case. Take one copy of the etching and with regular scissors, cut out the piece or pieces that needs to be replaced. When cutting, cut close to the center of the lead line that was traced in the original rubbing.
Once new pieces are cut and installed, re solder the once cut lead came joints. By doing it this way, it is impossible to tell that the window has been repaired! Then if the piece has a zinc frame, solder it back on before cementing.
Cement, clean and polish the window and it is now repaired!