Replace a broken window pane
Wearing gloves, remove as much loose broken glass as possible from the sash. If the putty holding the pane in place is old and brittle, you may be tempted to wiggle shards free, but it’s safer to chip away that putty with a narrow chisel — or sharpened flat-head screwdriver — then remove the broken glass. You’ll eventually have to remove all that putty anyway.
Embedded in the putty will be glazier’s points, little pieces of metal that held the window pane in place. Remove those with needle-nose pliers and discard them, too. Smooth out the sash with folded sandpaper or a sanding sponge.
Hire a professional
If you discover you’re dealing with an insulated window (one made of two panes of sandwiched glass), stop now and hire a professional. If it’s a single pane, proceed.
Order replacement glass that’s about 1/8 inch narrower and shorter than the opening but of the same thickness as the original pane.
Paint the rabbets (the tiny ledges the pane fits into) with linseed oil so the dry wood does not absorb the glazier’s putty.
Hold the window in place — or, better, have a helper do so — and push in new glazier’s points with a flat-head screwdriver or stiff putty knife about every 8 inches — but at least two on each side.
Roll a clump of glazing putty between your hands into a rope, then press that into the rabbet. Continue until you’ve gone all the way around window pane.
With a putty knife, press the putty firmly against the glass and tool to 45 degrees. Dipping the putty knife in linseed oil will make it move more smoothly. This finish step takes a little practice, but if you mess it up, you can remove that putty and start over. It will remain flexible for several days.
In fact, wait those several days before priming and painting the putty or the paint will crack as the putty shrinks.
Linseed oil, glazier’s points and putty and putty knives all are available from hardware stores and home centers — usually in the paint department.
By the way, people who install the glass in window sashes are called glaziers. I thought you ought to know that, too — now that you’re one of them.