Lay a large plastic tarp or old bedsheet below the window. Glass shards will scatter. Keep pets and children out of the room. Have a sturdy cardboard box ready for disposing of broken glass.
Having followed nearly identical steps on a drafty cellar window and a decorative casement, I can confirm the method works beautifully. The only variable is the condition of the old putty—if it’s pre-1970s and rock-hard, budget an extra hour for removal. Also, do not use this method on double-pane, argon-filled, or vinyl-framed windows; those are sealed units requiring replacement. how to repair a broken window pane