1. Watch and contain
Note what's happening, hold off further damage if it's safe to, and gather any background that matters.
2. Walk the house and lot
How the unit gets installed, the flashing detail, the condition of the existing sill, the frame material, the glass package, trim rework, and how much sun that wall takes on all carry as much weight as the window itself.
3. Put the scope in writing
Nail down prep work, protective measures, materials, access plan, permits, testing, cleanup, finish restoration, exclusions, and the rule for change orders.
4. Weigh the providers
Check credentials and look at the reasoning and limits behind each bid, not just the bottom-line number.
5. Close it out properly
Confirm what was tested, that cleanup is done, paperwork is handed over, warranty terms are clear, and who handles upkeep going forward.