Inspect for rot. If the wood is soft, dig out the rot and apply wood hardener or epoxy filler. Let it cure fully before proceeding.
Use a putty knife to scrape out any remaining old putty, dirt, or debris from the window frame. Clean the frame with a damp cloth to ensure a smooth surface for the new glass. how to repair a broken window pane
If you have a basic toolbox and patience, the DIY route is overwhelmingly worth it. The skill is easy to learn, and the satisfaction of repairing rather than replacing is genuine. Inspect for rot