Make AI write in your brand voice with a one-page voice guide

The fastest way to make AI write in your brand voice is to give it a one-page voice guide built from your own best writing, then save that guide inside a project (ChatGPT or Claude) or a Gem (Gemini) so every chat starts with it. Adjectives like “friendly” or “bold” won’t get you there on their own; real examples and a list of banned words will.

This playbook walks you through building the guide with AI’s help, the exact prompts, where to load it in each tool as of October 2026, and where it still goes wrong.

Why does AI copy sound generic?

Because without examples, the model falls back on its own default style. Ask for “a friendly, professional email” and you get the average of every friendly, professional email it has seen. That’s the flat, over-polished tone people now call AI slop.

A prompt that says “friendly but expert” gives the model almost nothing to copy. Five paragraphs you wrote, plus “never say ‘elevate’”, gives it a lot.

What you need before you start

• 5–10 pieces of your best writing. Emails that got replies, posts that did well, a sales page that converts, product descriptions customers mention. Pick by results, not by what you like reading.
• 2–3 pieces you hate. Competitor copy or old drafts that sound wrong for you. These help the model see the line.
• A chat assistant. ChatGPT, Claude or Gemini. The free plans are enough for this.
• About an hour. Most of it goes on reading the guide the AI drafts and cutting what isn’t true of you.

Strip out anything confidential first: client names, prices you don’t publish, personal details.

Step 1: Have the AI describe your voice back to you

Start a new chat, paste your best examples, then ask the model to tell you what it sees before it writes anything.

Below are examples of writing from my business that worked well.
Don't write anything new yet.

Describe the voice in these examples as specifically as you can:
- typical sentence length and how much it varies
- how pieces open and how they end
- words and phrases that keep coming up
- how formal it is, and how it uses humour (if at all)
- what it never does (e.g. exclamation marks, jargon, hype)

Quote the examples to back up each point. If something is inconsistent
across the examples, say so instead of smoothing it over.

[PASTE EXAMPLES HERE, each with a one-line note on where it was used]

Read the answer closely. If it comes back with “warm, engaging and professional”, it hasn’t found anything useful. Push back: “That’s too vague. Give me three things a reader would notice in the first two lines.” Good output names concrete habits, such as “opens with the customer’s problem in under ten words” or “uses ‘we’ for the business and never ‘our team’”.

Step 2: Turn it into a one-page guide

Now ask for the guide itself, in a fixed shape you can reuse.

Using your description, write a one-page brand voice guide with these
sections. Keep it under 400 words.

1. Who we write for (one sentence)
2. Voice in three plain phrases, each with a one-line "this means..." example
3. Do: 5-7 rules, each one specific enough to check
4. Don't: 5-7 rules, including any words or phrases we never use
5. Words we prefer -> instead of (a short table)
6. Two short before/after rewrites using my examples

Use British English. Don't add rules the examples don't support.

Then edit it yourself. Don’t skip this. Delete anything that’s flattering but not true, and add the rules you already apply without thinking: how you sign off, whether you use first names, what you call your customers.

Add these lines at the bottom whatever your voice is. They stop the most common problems:

Never invent statistics, customer quotes, reviews or results.
If a draft needs a fact I haven't given you, write [CHECK: what's needed].
Don't use these words: delve, elevate, unlock, game-changer, seamless,
in today's fast-paced world.

The [CHECK] rule matters most. Without it, the model fills gaps with plausible-sounding numbers, a hallucination that will go out under your name.

Step 3: Save the guide where the AI will always see it

Save the guide as a document called “Brand voice guide” and keep the master copy in your own files. Then load it into whichever tool you use.

Tool	Where it goes	Notes (as of October 2026)
ChatGPT	A Project: paste the guide into Project settings as instructions, and upload your best examples as files	Projects work on the free plan. OpenAI says project instructions apply only inside that project and override your general custom instructions
Claude	A Project: click Set project instructions for the guide, and add your examples to the project knowledge	Free accounts can create up to five projects. Everything in project knowledge is available to every chat in that project
Gemini	A Gem: paste the guide into the instructions and add your examples under Knowledge	If you add the guide from Google Drive, Gemini uses the latest version of the file. See the note below on skills

Why a project rather than your account-wide settings? Because you probably write in more than one voice. Your LinkedIn posts and your customer service emails shouldn’t sound the same, and a project per voice keeps them apart. Account-wide settings, such as ChatGPT’s custom instructions or Claude’s “Instructions for Claude”, are better for things that are always true about you, like British spelling.

A note on Gemini: Google’s help centre says Gems on personal Google accounts will switch to skills from November 2026 and convert automatically, with the same name, instructions and supported files. Workspace business accounts follow in March 2027. Nothing is lost, but it’s a good reminder of why the guide should live in a document you own, not only inside one tool.

Step 4: Test it on a real job

Open a new chat inside the project and give it something you’d normally write this week:

Write a 120-word email to past customers announcing [offer], ending
[date]. Follow the brand voice guide. After the email, list any place
where you weren't sure how the guide applies.

Compare the draft with something you wrote yourself. Then try a blind test: show both to a colleague and ask which one is AI. If they can tell straight away, ask what gave it away and turn that into a new rule.

Step 5: Fix the guide, not the draft

When a draft is wrong, it’s tempting to rewrite the sentence and move on. Next week you’ll fix the same thing again. Instead, ask:

I changed your draft. Here's the original and my edit.
What rule would have produced my version? Write it as one line
I can add to the brand voice guide.

Add the rule to the guide and update the copy in the project. Keep the whole thing under a page; when it gets long, merge the rules that overlap. A short guide is also easier for you to check a draft against.

Where does this break?

• Too few examples. With one or two samples, the model copies their quirks, including a phrase you only used once. Five to ten is a better spread.
• Mixed voices in one project. Put sales emails and casual Instagram captions in the same project and you’ll get something in between. Split them.
• Long pieces. Watch the second half of anything over a few hundred words for the default style creeping back. Write long pieces in sections, or ask the model to check its draft against the “Don’t” list before it finishes.
• It can’t know facts. A voice guide controls how things are said, not whether they’re true. Prices, dates and claims still need checking by a person.
• Free-plan limits. File caps and usage limits are tighter on free plans. A one-page guide plus a handful of examples fits, but don’t upload your whole archive.

How much time does it save?

Setting it up takes about an hour. The payback is in editing: instead of rewriting AI drafts from scratch, you’re correcting a few lines. How much that saves depends on how much you write, so time your next five drafts with and without the guide and see for yourself.

What to do next

Put your voice guide to work on something you’ll publish this month. The ChatGPT social media content calendar playbook uses the same project set-up to plan 30 days of posts, and it’ll read much more like you with the guide loaded. If you’re rolling AI out across a team, the AI capability assessment helps you see who needs help first.