How to Build a Custom GPT in Someone's Voice
I've spent a good part of my writing career as a ghostwriter and as a business journalist. Ghostwriting is common because many founders hate to write, don't know how to write about themselves, or simply don't have the time. Often, they’d much rather pay a skilled writer to do it for them just like they do with everything else. Entrepreneurs and CEOs value their time more than anything, therefore they will hire experts to do what they don't want to do.
Over the years, I've had many incredible opportunities to write for extremely successful founders, mostly about how they built their businesses from scratch. But you'll never see my name on any of those articles.
I’ve emotionally watched grown men cry. Often, the same themes would arise in those candid conversations. You know, those "fetal position moments," how critical it is to have a supportive partner, the importance of hiring for culture fit over resumes, and how it's often a bad idea to work with family members—actually, most of their kids don't want to work for them anyway.
Initially, ghostwriting meant a phone call where I'd type what the founder said at lightning speed in real time while I conducted my interview over the phone. Then, I progressed. I'd conduct a one-hour Zoom interview, record it on my cellphone, then spend three hours transcribing it.
I had my first fetal position moment when I interviewed a very successful digital creator who took months to get an interview with, and my iPhone glitched, erasing the entire interview. After I finished screaming, "Noooooo!" and convulsing on my office floor, I ran over to my computer and quickly typed up everything I could remember from that interview.
I spent the next couple of hours conducting a full internet search to fill in the gaps. Shockingly, she approved the article, but that was the last time I ever relied on my voice memo app again. I immediately found Otter.ai, which records and transcribes meetings, and my life was turned upside down in the very best way. That $99-a-year subscription gave me 40 hours of my life back each month thereafter.
Fast forward to the release of ChatGPT in November 2022. By this point, I had been writing professionally for about 12 years. After all, this is how I supported my three children. It was around 2024 or 2025 when ChatGPT was becoming a household name and for the very first time in my writing career, I felt threatened. I worried, "Would I be replaced by artificial intelligence?"
At the suggestion of my colleague (thank you Brayden), I agreed to look into AI to see how it could help me "be more productive." Admittedly, I went down the rabbit hole for six months straight teaching myself, learning the ins and outs of ChatGPT and writing, finding its weaknesses, finding the holes, learning about "hallucinations," and so on.
My official conclusion? I realized AI was nowhere near replacing journalists and writers. I breathed a sigh of relief. I felt safe, but after playing with AI for months, I also realized I could never complete a writing project without it ever again. I was transformed. The writer who went down the rabbit hole emerged from the other end as someone with a highly evolved skill set.
Hooked, I began building custom GPTs (Generative Pre-trained Transformers, a class of large language models, or LLMs, developed by OpenAI). One after another. I couldn't stop. I kept thinking of systems and workflows that AI could improve, especially when combined with my years of subject matter expertise.
I quickly learned that the "garbage in, garbage out" theory is 100% real when it comes to prompt engineering and building fine-tuned GPTs in ChatGPT or "Gems" in Gemini. A GPT is only as good as the person building it.
Along my AI journey, I learned how to use ChatGPT to build a custom GPT in anyone's voice—this is also called persona modeling, a brand-voice GPT, or a "persona-based GPT." In other words, you train the GPT to sound like a particular person.
Such GPTs can be used by marketers, startup founders, entrepreneurs, and businesses of all sizes for email marketing, social media posts, newsletters, and writing website copy in someone's voice and tone, usually the founder's or CEO's voice, which is often the same person. There is one caveat—the person building the GPT needs to understand that person very well, including their distinct voice and tone.
What You Need for a Persona-Based GPT
If you want to build such a GPT, at the bare minimum, you'll need ChatGPT's Plus plan (around $20 a month). As of this writing, you can't build custom GPTs with the free plan, but you can with the Plus, Pro, Business, and Enterprise plans.
To build a persona-based GPT, you will need PDFs to build the GPT's knowledge base. To do this, you can use articles, newsletters, emails, or blogs the person has written. Since ChatGPT currently has a limit of 20 PDFs that can be saved in any one GPT, I recommend copying and pasting as many articles from the person as you can onto one Google doc, then saving it as a PDF.
I highly recommend adding transcripts from recorded Zoom meetings, Microsoft Teams meetings, Google Meet sessions—anything where the individual has been recorded and that transcript has been saved as a PDF, whether exported from Zoom or Otter, or saved in a Google or Word document and saved as a PDF.
You can also use transcripts from podcasts or YouTube videos the person has been on. If the YouTube video doesn't have the transcript on the channel, you can download Otter for $99 for the entire year on your phone and laptop or PC, play the video and record it on Otter (or a similar AI platform), via your phone or PC, then export the transcript to your desktop.
Essentially, you want to get as many articles (if available), newsletters, blogs, or transcripts of the person talking or writing so you can upload those PDFs into the GPT's knowledge base.
If he or she has certain phrases they use often, you can write a list of these common phrases in a Google doc, save it as a PDF and add that to the knowledge base. The idea is to "train" the GPT to sound like the person.
If you have examples of articles, emails, and the like to train it, then that's optimal. If you have nothing like that, you can record the person talking, even at meetings or giving company speeches, and use those transcripts (from something like Otter.ai) to build the GPT. You can even find a dozen or more emails they've sent, compile them in a document, save it as a PDF, then upload that to the GPT.
Building the Custom GPT
Once you have your PDFs for your knowledge base, now comes the fun part. To start building in ChatGPT, go to "new chat." Here, you're going to let ChatGPT build its own instructions for your GPT. Tell it exactly what you want it to do. Act as if you're telling someone like me to build the GPT for you. Here’s what I did for a fictitious CEO…
My Prompt (to ChatGPT)
I want you to write instructions so I can build a custom GPT in the voice and tone of John Doe, founder of Yummy Tummy Cookies. I want the GPT to write emails, social media posts for Instagram, X, Facebook, and LinkedIn, newsletters, blogs, articles, etc. in the voice and tone of John Doe using the knowledge I provide you.
Instructions:
You are the founder of the biggest cookie franchise in the U.S. You are knowledgeable about cookie recipes. Write in a fun, down-to-earth fashion. As an expert on cookies, you talk about ingredients and flavors. You write from a 10th-grade reading level. Write with passion and excitement for cookies.
Avoid:
Avoid foul language.
Avoid chunky paragraphs.
Avoid technical writing.
I typed in the above, and ChatGPT created its own instructions for the GPT:
Custom GPT Instructions Voice & Tone: John Doe, Founder of Yummy Tummy Cookies
You are John Doe, founder of Yummy Tummy Cookies, the largest cookie franchise in the United States. You are the voice, personality, and creative mind behind the brand.
Role & Perspective: Write in first person as the founder. Speak with the confidence of someone who built a massive, beloved cookie brand from the ground up. You are an expert on cookies, flavors, and ingredients—but you explain things simply and warmly. You genuinely love cookies and want other people to feel that joy.
Writing Style & Tone: Fun, upbeat, and down-to-earth. Passionate and enthusiastic about cookies. Friendly, conversational, and approachable. Reads at a 10th-grade level. Sounds like you're talking to a real person, not a corporation.
Content You Create: You write emails, Instagram captions and reels text, X (Twitter) posts, Facebook posts, LinkedIn posts, newsletters, blog posts, articles, and brand storytelling content. All content must stay consistent with the John Doe/Yummy Tummy Cookies voice.
Brand Voice Guidelines: Talk about ingredients, flavors, textures, and smells in an exciting, mouth-watering way. Share behind-the-scenes thoughts, founder insights, and passion for quality. Keep paragraphs short and easy to read. Use simple language that feels natural and human. Let your excitement for cookies come through in every piece of writing.
Reading & Formatting Rules: Short paragraphs. Clear, punchy sentences. No walls of text. Easy to skim.
Avoid: Foul or vulgar language. Chunky or dense paragraphs. Technical or overly scientific explanations. Corporate jargon. Overly formal or stiff writing.
Knowledge: Use only the knowledge provided to you unless otherwise instructed. Do not invent facts, recipes, or business claims unless explicitly asked. Stay aligned with the Yummy Tummy Cookies brand values at all times.
Core Mission: Your goal is to make people smile, make people hungry, make people feel connected to the brand, and share genuine excitement for cookies and the joy they bring. Always write like someone who loves cookies and loves sharing them with the world.
Next Steps
Copy and paste the output (the GPT instructions) from ChatGPT onto a Google doc or Word document. Read over it closely and tweak it where you see fit.
To now build the GPT, click on "Explore GPTs" on the left sidebar in ChatGPT.
On the top right, click "Create." Then, on the left side of the screen, ChatGPT will ask, "What would you like me to make?" You will see "Create and Configure." Make sure you are under "Create” and copy and paste the instructions ChatGPT gave you earlier.
ChatGPT will ask you to name the GPT. Go ahead and name it. ChatGPT will generate an image for your GPT. You can tweak it if you don't like it. Next, click on "Configure," which will be directly to the right of "Create."
Under "Create," look over the description, instructions, and conversation starters and tweak them until you are happy with them.
Under "instructions," you will return periodically to tweak or update the GPT if necessary.
Under "Knowledge," upload your files. This will be any articles, emails, blogs, meeting transcripts, podcast transcripts, etc. of the founder or CEO. You are using these to build the GPT in his or her voice and tone. Once everything looks good, on the top right side of the screen, click "Create." You will be given the option to share the GPT or keep it private.
Congratulations! Once you have completed the above steps, you have created your custom GPT in the voice and tone of a particular person. Now what you want to do is play with it and keep fine-tuning it until it does what you want it to do.
At any time, you can edit the GPT and add, tweak, or delete instructions. You can also swap out PDFs or add to the knowledge base, but just remember that as of this writing, ChatGPT only lets you upload 20 PDFs.
Fine-tuned GPTs are incredible. Not only are they great for writers, marketers, and companies as a whole, but anyone can build a GPT in their own voice to help them draft social media posts, email campaigns, website copy and more, even if the person has difficulty writing because they get writer's block or struggle with writing in general. Custom GPTs are time-saving tools that can help marketers and founders 10x their productivity.
