Have the new glass cut so it measures 1/8 inch shorter than the exact groove dimensions in both the length and width. This creates a 1/16-inch gap on each side between the edges of the pane and the rabbet groove. The gap provides room for the glass to expand when the weather changes.
In addition to the replacement glass pane, you'll need: latex glazing putty; metal glazing points; pliers; a heat gun; flexible putty knife; 1- or 2-inch, stiff, steel, putty knife; and a flathead screwdriver.
Use the pliers to remove all of the glass shards.
When you work with broken glass, wear safety goggles as well as gloves; small chips of glass can cause permanent eye damage.
Use the heat gun to warm the old putty; and then scrape it away with a putty knife.
If the putty doesn’t lift off easily, apply more heat and try again. Be patient — the putty around really old windows is as hard as concrete, but it will soften.
Don’t chisel out the old putty — you might wreck the window. The putty will soften with enough heat and time.
Use a putty knife or the tip of a screwdriver to remove the old glazing points.
The points will be buried under the old putty.
Clean and inspect the rabbet groove.
Ensure that no glazing putty, glass shards, or glazing points remain.
Apply a bead of putty on the glass side of the rabbet groove.
Squeeze out a 1⁄16-inch bead of putty between the edge of the glass and the window frame.
Gently press down the glass at the edges to embed the glass into the putty.
Allow the putty to spread out and form a moisture seal on the inside of the window between the glass and sash.
Position the new pane in the rabbets.
Shift the pane until you have a 1/16-inch gap between the pane and sash on all four sides.
Place at least two new glazing points in each section of the window sash surrounding the new glass.
Space the points evenly around the perimeter, about 6 inches apart.
Use the flat side of a putty knife to push one corner of each triangular glazing point into the wood sash.
You can use a screwdriver blade instead of a putty knife.
Form a 1/2-inch-thick rope of putty and press the length along all four sides of the glass.
Roll a glob of putty between your bare hands to form the rope.
Smooth the glazing putty and scrape away the excess.
Hold the putty knife at a 45-degree angle, press, and smooth the glazing putty against the glass and sash.
After the putty dries completely, repaint the putty and repaired area.
Don’t mask off the glass before repainting. The paint will help form a moisture seal between the glass pane and the sash. So, allow the paint to overlap about 1/8 inch onto the glass.