1. Note it, then contain it
Record the symptoms, limit further damage if it's safe, and gather relevant background.
2. Look at the house and the lot
Installation approach, flashing, sill condition, frame material, glass package, trim rework, and how much sun the wall gets under the oak canopy or in the open all matter as much as the window itself.
3. Get a written scope
Preparation, protective measures, materials, access, permits, testing, cleanup, finish restoration, exclusions, and change-order handling all need to be spelled out.
4. Compare the proposals
Check credentials and weigh the reasoning and limits behind each bid, not simply the total.
5. Wrap up properly
Confirm testing, cleanup, paperwork, warranty terms, and who's responsible for maintenance afterward.