Take a room-wide photograph
Show the stain, ceiling and nearby chimney or chase location. A close-up helps record the mark, but the wider view explains where it is.
Add safe exterior views
Photograph from the ground where possible. Do not climb onto a roof or lean out of a window for this request.
Write down the weather
Note when the leak appeared, whether wind was involved and whether the condition is recurring. Do not assume the timing proves which detail failed.
Collect prior repair information
Roofing, sealant, flashing and chase work may each be relevant. Send existing reports privately and ask for clear source-review and finish-restoration responsibilities.
Photos help start the review. Access, concealed conditions and required professional review can change the construction scope.