How should a California tenant document move-in damage?

Write down every existing defect on a move-in inspection checklist on the day you take possession of the unit, before unloading any of your belongings. Photograph every wall, floor, appliance, fixture, and any pre-existing damage in detail. Date-stamp the photos. Have the landlord (or property manager) sign the checklist; if the landlord refuses, send a copy of the completed checklist by email to the landlord the same day with the photos attached. California small-claims courts treat the move-in checklist as the baseline for what the tenant did or did not damage. Without a documented baseline, every disputed move-out deduction becomes a he-said-she-said. The first 24 hours of a tenancy are the single most important window for protecting the deposit at move-out. Up to twice the wrongfully withheld amount if the court finds bad faith. Attorney fees also recoverable.

Source: Cal. Civ. Code 1950.5(h)(1)