Guidelines are usually written at the end of an identity project, by the people who know it best, for an audience that already understands it. Then they are handed to a printer, a freelancer or a new marketing hire who has none of that context.
Explain the why, briefly
A rule without a reason gets broken the first time it is inconvenient. “Keep the logo clear of other marks” is followed more often when it adds “because it has to stand alone to be recognised at small sizes”. One line of reasoning per rule is enough.
Show more than you describe
A paragraph on tone of voice is useful. Five real examples of the voice in a sales email, an error message and a social post are more useful. The same goes for layout, photography and colour: show correct and incorrect use side by side.
Cover the dull cases
Guidelines tend to show the brand at its best: the hero poster, the launch film. Most of the brand's life is spent on invoices, email signatures, slide templates and job ads. Those are where it drifts, and they deserve pages of their own.
Ship templates, not just rules
The most reliable way to get a slide or a social post right is to start from a file where it is already right. Templates for the ten most common formats will do more for consistency than another fifty pages of rules.
Keep them alive
A PDF is out of date the week after it is sent. A simple web page that can be updated, with a named owner and a date on it, stays useful. When a question comes up that the guidelines do not answer, the answer should be added to them.
