Pull the current brand guidelines, the live site, and the last ads that were actually approved — not a moodboard from two rebrands ago. Write down the assets you will load: primary logo, clear-space rule, hex colours, the one or two fonts you will allow on captions, and any words legal has already banned. If two documents disagree, the live site wins until a human owner says otherwise.
Most 'brand kits' fail because they are a dump of every file in a shared drive, including retired logos and a display font nobody can read at 9:16. You need a short list: one primary logo for dark backgrounds, one for light, a minimum size, hex values for background, text and accent, and the caption font that still reads at arm's length on a phone. If the official typeface is a thin display font, do not use it for burned-in captions — pick the allowed sans-serif from the same guidelines and note the exception in writing. UGC has to read in a feed, not on a poster in a lobby.
Ask whoever owns legal or the founder which claims are approved, which need a qualifier, and which are never to appear in a hook. That list belongs in the kit, not in someone's head or in a Slack thread from last spring. A kit with beautiful colours and no claim rules will still ship ads that get rejected or, worse, spend for a week before a takedown. Ten minutes with the live PDP, the latest legal notes, and one approved ad is cheaper than twenty re-renders after a reviewer finally looks at file twelve.