1. Note it, then contain it
Write down the symptoms, limit further damage where it's safe to, and pull together any history on the windows.
2. Take stock of the house and lot
Sill condition, flashing, frame material, glass package, trim restoration, and how much sun a wall gets all matter as much to the outcome as the window itself.
3. Get everything in writing
Spell out preparation, protection measures, materials, access plan, permit responsibility, testing, cleanup, finish restoration, exclusions, and how change orders are priced.
4. Weigh more than the total
Check each provider's credentials and understand the reasoning behind their scope — the bottom-line number alone doesn't tell you much.
5. Finish with a proper closeout
Confirm testing was done, the site was cleaned, documentation and warranties are in hand, and you understand ongoing maintenance responsibilities.