I just found it. What comes first?
Capture, before anything else. Not the report button, not a reply, not a message to friends. Posts get deleted, edited in place, made private or replaced by a fresh upload with a different address, and every one of those actions destroys part of what you would need later.
Download the file itself in the form it was published in. Calculate a checksum straight away, so that the copy on your machine can later be shown to be unaltered. Take full screenshots that include the caption, the visible engagement numbers, the account that posted it and the address bar, rather than cropped fragments.
Write down the time you found it and include the time zone in words, because the platform desk reading this may sit on the other side of the world. A screen recording of the clip playing preserves the picture and the sound exactly as an ordinary viewer received them.