What goes in a brand voice kit
A short description of your audience and what they care about. Three to five tone rules, such as 'plain English, no hype'. A list of banned words and claims. Approved facts and proof points with sources. Several examples of on-brand writing, and a few rewritten off-brand examples. Visual rules for images and video.
Keep it current
Update the kit when your services, prices or positioning change, otherwise AI will keep repeating old claims. Collect the edits reviewers make and add the patterns to the guide, so the same corrections are not needed twice.