Start before you touch anything
Walk the area and photograph it as you found it, including any damage that was already there. Marks on a wall, a cracked tile, a scuffed floor: photograph them before you start and nobody can attribute them to you later.
Photograph anything that gets covered up
Pipework, cabling, fixings, membranes, insulation. Once the plasterboard goes on, the photo is the only way anyone will ever see it again.
Say what the photo shows
A photo on its own can be ambiguous. Ten seconds of voice explaining what it is and why you took it turns it into a useful record.
Keep it filed by job
The record is only useful if you can find it. Filing photos and notes under the job as you go saves you scrolling through a camera roll months later.