Tools needed: new glass, glazier's tool (or flat-head screwdriver and putty knife) gloves, safety glasses, pliers, measuring tape, glazier's points, mineral spirits (for cleanup)
The first step is removing the broken glass. Wear work gloves and safety glasses while removing the broken pieces of glass from the frame. A shattered windowpane can be removed quite easily. Pull one broken piece out at a time. If the glass is only cracked, you may need to remove most or all of the glazing compound in order to take out the broken glass.
Next, remove the old glazing compound from the window frame. A metal putty knife works great for this. As you're removing the old compound, you may notice small metal triangles stuck in the wood. These are the glazier's points, which actually hold the glass in the window. Using pliers, carefully remove and discard all old points.
Using a measuring tape, we need to get the length and width for our new glass. You'll be measuring the space the glass will fill, not just the visual opening. Make sure you include the shelf where the glass will lay.
After you have removed the old glazing compound, apply a very thin layer of new compound completely around the frame where the new glass will be set. This will act as a cushion for the new glass, and will stop air leakag. You can now insert the replacement glass in the frame. Carefully insert the new windowpane into the frame carefully. Press it down firmly.
Using a screwdriver or the flat side of your glazing tool, insert a glazier's point on each side to secure the pane firmly in place. Lay each glazier's point flat against the glass and start it into the wood with the point of the combination glazing tool or putty knife. Then, use the glazing tool to drive the glazier's points into the wood.The glazier's points can be inserted with only a small amount of pressure. Insert additional glazier's points about every 4" apart, completely around the new pane of glass.
Glazing compound should be the consistency of rather dry, thick dough when it is applied. Knead the putty until it is completely pliable and free of lumps. Then roll it into pencil-size strips. Take a roll of compound into your hands and start applying it in one corner of the window frame. Lay the strip in the frame completely around the new piece of glass. When the compound is completely in place, smooth it out with a putty knife or scraper/glazer using long, even strokes. Whether you use a putty knife or a scraper/glazer, it may be difficult at first. However, with a little practice, you can smooth out the compound completely around the windowpane. Keep smoothing until the compound takes on a neat, finished appearance. Scrape completely around the area, and remove any excess in corners or along the edges.
Because glazing compound is oil-based, you may need to clean up excess with mineral spirits. Otherwise, any excess will need to be cut or scraped off when it dries. Follow manufacturer's directions. Comound may need to set 7 - 14 days to form a skin and firm up. That will depend on temperature and humidity. Once it has cured, you can now paint the glazing compound for a final seal.