Measure the window across the outer edges of the surrounding stop. A stop is a piece of trim that holds the pane of glass in the door. Subtract 1/8 inch from the total. If the glass isn't broken, you can measure the pane itself after you remove the stop.
Remove any screws or nails securing the stop in place. Scrape the layer of glazing compound from the perimeter of the glass and remove the pane.
Fit a caulking gun with a tube of glazing compound and apply a bead of glazing compound around the perimeter of the opening. Place several stop beads against the edge of the door, over the compound.
Place the glass in the opening and press it into the glazing compound.
Nail or screw the stop back into place.