Soften the glazing around the perimeter of the broken glass with a heat gun.
Remove remaining glass and glazing points, which are small metal pieces that hold the glass in place while glazing, with needle-nose pliers. Set the glazing points aside. Replace if they are damaged.
Scrape off remaining putty with a 1½-inch putty knife. Keep the blade at an angle and go slowly to prevent damaging the frame.
Measure opening with a tape measure.
Cut glass with a glass cutter. Mark the measurements for each direction, vertical and horizontal, on each side of the glass. Position a straight edge across the shortest side and run a glass cutter down and back with evenly applied pressure. Position the score 1/8-inch past the edge of a table and apply firm and steady pressure to press down on the area to be cut off. If the pressure was consistent in both directions, the glass will break exactly on the score line. Repeat for the other side.
Warm glazing in your hands. Press a 1/8-inch layer of glazing on the back of the window opening.
Position the glass in the frame. Press into the glazing in the frame. Use needle-nose pliers to replace glazing points; one on each end and two on each side.
Warm glazing in your hands and apply a ¼-inch thread of glazing around the perimeter of the window. Press into the corner with a 1½-inch putty knife held on a 45-degree angle. Maintain a ½-inch border of glazing around the glass. Allow to dry until firm, about an hour.
Trim lumps in glazing with a utility knife.