How to Build Custom GPTs

Glen Pfaucht
May 8
5 min read

Updated: May 30

A Practical Guide to Designing Useful, Purpose-Built GPTs

If you’ve experimented with ChatGPT, whether generating content, answering questions, or summarizing information, you’ve only scratched the surface of what it can do. The real value emerges when you create Custom GPTs: purpose-built versions of ChatGPT tailored to perform specific tasks. These customized bots retain the knowledge and instructions you provide, allowing them to deliver consistent results without the need to repeat prompts every time.

In this guide, we’ll walk you through the step-by-step process of building your own Custom GPT from start to finish.

What Are Custom GPTs?

A Custom GPT is a personalized version of ChatGPT that behaves according to your specific instructions. You can shape its identity, behavior, and knowledge, enabling it to operate like a specialized assistant or domain expert.

These GPTs can respond in your preferred tone, follow custom rules, and even access documents or APIs. Whether you’re building a support agent, copy assistant, coding tutor, or strategic advisor, Custom GPTs can streamline workflows and deliver consistent, accurate responses.

Note: Only ChatGPT Plus users can create and share Custom GPTs. However, non-subscribers can still use them once shared.

Related: Is ChatGPT Plus Worth It?

Step-by-Step: How to Build a Custom GPT

1. Access the GPT Builder

Go to chatgpt.com/gpts and click the Create button.

You’ll see three tabs:

Create – an AI-guided setup interface (not recommended).
Configure – the core settings area where you define the GPT’s identity and behavior.
Preview – a live test environment to refine how your GPT responds.

I recommend skipping the "Create" tab and working directly within configure, which offers greater control and clarity.

2. Configure Your GPT

The Configure tab allows you to define key characteristics:

Name – Use a clear, concise title that reflects your GPT’s function. Avoid vague or gimmicky names.
Description – Provide a short summary of what the GPT does and who it is for.
Instructions – This is where your GPT’s core behavior is defined (see the INFUSE framework below).
Conversation Starters – Predefined user prompts that help demonstrate capabilities.
Knowledge Files – Upload up to 20 files (512MB each) for reference. These can include examples, documentation, or structured data.
Capabilities – Enable tools like Code Interpreter, Web Browsing, Image Generation, or Canvas depending on your use case.
Actions (optional) – If you want your GPT to make API calls, integrate external tools, or fetch real-time data, define custom actions here.

3. Structure the Prompt: The INFUSE Method

Unlike a regular ChatGPT prompt, your Custom GPT's system prompt acts as a permanent instruction set. It must define the GPT’s persona, use of knowledge, tone, and how it interacts with users. Here are some frameworks that provide a reliable method for writing these.

Identity & Goal - Define the GPT’s professional role and core objective. Example: “You are a financial consultant helping users evaluate risk in business plans.”
Navigation Rules - Specify how the GPT interprets commands and when to use knowledge files or tools. For instance, "When a question involves data from uploaded files, refer to those files directly."
Flow & Personality - Determine tone (e.g., professional, instructive, concise), language level, and any stylistic preferences.
User Guidance - Explain how the GPT should assist users. For example: “Ask clarifying questions if a query is unclear, and always provide a clear summary of recommendations.”
Signals & Adaptation - Guide the GPT to adapt based on user input. If users express confusion, it should simplify its responses. If urgency is detected, it should prioritize actionable solutions.
End Instructions - Include any final non-negotiable rules, such as compliance limitations or mandatory response structure (e.g., "Always end with a key takeaway summary")

Using Knowledge Files Effectively

Knowledge files serve as an extension of your GPT’s core instructions. They essentially function as a secondary layer of context. When used effectively, they enable your Custom GPT to produce more accurate, detailed, and contextually appropriate responses.

Rather than uploading large amounts of raw or unstructured content, the goal is to curate information that supports the GPT in achieving a clearly defined objective. Think of these files as reference material, not a data dump.

For example:

If you're building a GPT for writing emails, include past messages that reflect the tone, format, and voice you want it to emulate.
For a coaching-focused GPT, share transcripts of real coaching sessions to help it learn how to ask insightful questions and respond with empathy.
If the GPT is intended to solve complex problems, upload structured case studies or detailed walkthroughs that it can reference when guiding users.

The prompt must clearly define when and why the GPT should refer to those materials. Without this guidance, the GPT may ignore your files or misapply them.

In short, well-organized, purpose-driven knowledge files combined with clear instructions are what transform a GPT from generic to highly specialized and effective.

Best Practices:

Include only high-quality, well-structured content.
Label files clearly (e.g., "EmailTemplates.pdf", "CoachingScripts.txt").
Reference them explicitly in your prompt. Example: “When writing emails, reference the file ‘EmailTemplates.pdf’ for tone and structure.”

Advanced Behavior: Signal-Based Adaptation

To make your GPT more responsive and human-like, you can teach it to recognize signals and clues from user input that suggest emotional state, confusion, or other context.

How to Implement Signal Responses:

Create a file called Signals.txt
List 20–30 common user cues and suggested responses
- Example Signal: “User expresses doubt”
- Suggested Response: “It’s perfectly reasonable to feel unsure at this stage. Would you like me to break it down further?”
In your prompt, add:“Adapt your tone and behavior based on user intent, referencing the Signals.txt file when appropriate.”

Avoid putting examples directly into the prompt to reduce clutter and maintain performance. Instead, rely on the file-based structure.

Testing and Refinement

Once your GPT is configured, move to the Preview tab and test it extensively. Your checklist should include:

Does the GPT follow its defined identity and tone?
Are knowledge files referenced appropriately?
Do responses feel natural, helpful, and consistent?
Is it able to adapt when users are vague, confused, or emotional?

Custom GPTs often require multiple iterations. Adjust the prompt, reword instructions, or re-upload revised files as needed. Small improvements can significantly increase the GPT’s effectiveness.

Naming & Presentation Guidelines

When naming your GPT:

Keep it professional, short, and relevant.
Avoid using brand names or personal names without permission.
Refrain from clickbait or inappropriate terms.
If part of a broader service, align the name with the parent brand (e.g., “Tracy Support” for customer assistance).

A well-named GPT is more discoverable, trustworthy, and appealing to users.

Final Thoughts

A well-designed Custom GPT is a tool that reflects your expertise, brand, or workflow. With a well-written prompt, strategic use of knowledge files, and thoughtful iteration, you can create GPTs that stand out for their clarity, usefulness, and professionalism.

The process takes effort, but the payoff is clear: better automation, smarter interactions, and a more engaging AI experience for everyone who uses it. And for any help with this process or any other AI/marketing things, at Open World Digital we're always available.