If the glass is loose and broken, put on gloves and safety goggles and remove the pieces, carefully working from top to bottom. If it is cracked, crisscross the pane with masking tape and remove the sash from the window opening. Place the sash on a work space covered with something soft (like a towel). Score the cracked glass with a glass cutter and then tap out the sections with the handle.
Scrape the glazing compound from the grooved channel on all sides of the sash with a putty knife, being careful not to gouge the wood. Also remove the glazing points with pliers. Coat the bare wood with a fast-drying primer.
Measure the width and length of the grooved channel to determine the size of the replacement pane. Subtract 1/8″ from both measurements so that the replacement pane will be slightly smaller than the opening.
Roll a narrow rope of glazing compound, and fit it into all sides of the grooved channel. Then place the replacement pane back into the sash.
At 6″ or 8″ intervals, lay a glazing point on the window with its tip facing the sash, and press it into place with the putty knife. The back of the putty knife can be tapped with a hammer to secure the points, but don’t directly hammer them.
Cover the points with a rope of glazing compound or a bead of caulk. Trim the compound with a putty knife for a smooth finish. Allow the compound to cure before painting. Make sure the paint overlaps the pane by 1/8″ for a weathertight seal.