Inspect your window before starting the removal process. Notice where the glazing has, essentially, started chipping away by itself; plan to start removal here–usually where the putty meets the wood, not where it meets the glass. Maintain a steady work surface if the window is not in place.
Use a putty knife, paint scraper, utility knife, chisel or any combination of tools that works well for you, to remove the putty, concentrating first on areas where the putty is deteriorated and/or missing. Carefully scrape, pry and work at removing the glazing, taking care not to break the glass if it is still present in the window. Work a small section at a time until you come to a push point, sometimes called a glazing point. Spaced every 6 to 8 inches generally, these points hold in the glass.
Avoid loosening the glazing points as you work, and don’t dig your knife or tool into the wood frame. Use a heat gun or similar device to carefully heat the window glazing slightly to help soften and loosen it for removal if needed. Sand with sandpaper to finish the glazing removal, and provide a fresh new surface for your new window installation.