1. Look, then protect
Note what's wrong, keep it from getting worse if it's safe to do so, and gather whatever history you have on the windows.
2. Assess the house itself
How the frame gets set — flashing details, sill condition, the material chosen, the glass package, trim work, and which way the wall faces the sun — shapes the outcome as much as the window itself.
3. Put the scope in writing
Nail down prep steps, what gets protected, exact materials, how crews will access the work area, permit responsibility, testing, cleanup, finish restoration, exclusions, and how changes get priced.
4. Weigh the proposals
Look past the bottom-line number — check credentials and understand why each provider scoped the job the way they did.
5. Close the job out properly
Before calling it done, confirm testing was performed, the site was cleaned up, you have documentation and warranty paperwork, and you know what maintenance is on you going forward.