How to build a brand voice guide with AI in one sitting

TL;DR: Do not ask AI to invent a voice. Feed it 10-15 pieces you have already written that felt right, have it reverse-engineer the pattern, then convert that into rules specific enough that someone else could follow them.

Why most voice guides fail

“Friendly, professional, approachable.” Every brand says this and it constrains nothing. A voice guide is only useful if a freelancer who has never met you could use it to write something that sounds like you — and if it can tell you what not to write.

Step 1: Collect the good stuff (20 minutes)

Gather 10-15 samples of your own writing that you were happy with — emails, posts, pages, replies. Also gather 3-5 samples you disliked. The contrast is more informative than the positives alone.

Step 2: Reverse-engineer the pattern

“Below are 12 pieces I wrote that feel like my voice, and 4 that do not. Analyse the difference concretely: average sentence length, how paragraphs open, use of contractions, use of questions, technical vocabulary level, how humour appears, how I handle uncertainty, what rhetorical moves recur. Give me measurable observations, not adjectives. Explicitly name what is present in the good samples and absent from the bad ones.”

Claude tends to be strongest at this kind of structural analysis — see the editing tools comparison for why.

Step 3: Convert observations into rules

Turn each observation into something checkable:

• Not “conversational” but “use contractions always; average sentence under 18 words; at least one sentence under 6 words per paragraph.”
• Not “confident” but “state the conclusion first, then the reasoning. Never open with ‘In today’s fast-paced world.'”
• Not “human” but “reference a specific real experience within the first three paragraphs.”

A rule you cannot verify compliance with is decoration.

Step 4: Build the banned list

This is the most practically useful section and almost nobody writes it. List the words and constructions that immediately read as not-you. Mine includes: delve, landscape, tapestry, “it’s not just X, it’s Y”, “in an era where”, em-dash-heavy sentences stacked three deep, and any sentence beginning “Let’s dive in.”

Banned lists are effective because they are unambiguous. A reviewer can check them in seconds.

Step 5: Add three worked examples

For three common formats — a social post, a client email, a section intro — include a bad version and a good version side by side, with a one-line note on the difference. Examples teach faster than rules; this is the same reason few-shot prompting outperforms instructions, as covered in the fine-tuning explainer.

Step 6: Turn it into a system prompt

Compress the guide into 10-15 rules and paste it into your custom instructions or a Project. Now every draft starts closer to your voice by default. The mechanics of that are in the system prompt explainer.

One caution: a voice guide makes AI drafts closer to your voice, not identical. Keep the final pass human, especially for anything with a point of view. The most valuable part of your writing is the part no guide can specify — the specific thing that happened to you.