Buy a pane of glass that is 1/16 in (1.55mm) smaller that the window opening on all sides. Chip out any old putty. Clean out the recess, and paint it with primer. Lay a bed of fresh putty in the rabbet in the window frame. Press the new piece of glass into position around the edges – not in the center, where it could break. Insert glazing brads or clips to keep the pane of glass in place.
Here's how to reglaze those old broken window panes. First, remove the old glazing compound with a propane torch and putty knife. Carefully remove the old glass and clean the (groove) with a soft wire brush. Apply a thin bead of backglazing, replace the glass, and gently place the push points every eight inches to hold the pane in. Apply a generous bead of fresh glazing compound and smooth it with your putty knife.