How to Build Custom Instructions That Actually Capture Your Voice

Custom instructions tell AI how to write like you. They're ChatGPT's built-in personalization—and they work. The problem is knowing what to put in them.

Most people write vague instructions like "be professional but friendly" because they can't articulate their own patterns. Style Profiles solve this. They're the comprehensive content you paste INTO your custom instructions—extracted systematically from your actual writing instead of guessed manually.

This guide breaks down what comprehensive custom instructions should include, why most people's instructions fall short, and how Style Profile extraction generates the detailed content you need.

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What Custom Instructions Actually Do

Custom instructions are persistent directives you set once in ChatGPT (or system prompts in Claude). They apply to every conversation going forward.

They're useful. They're free. And for many use cases, they're sufficient.

What custom instructions handle well:

• Basic tone direction ("formal," "concise," "friendly")
• Output format preferences (bullets vs. paragraphs)
• Role and context reminders ("I'm a product manager")
• Simple rules ("never use exclamation points")

What custom instructions require from you:

• Self-awareness about your patterns
• Ability to articulate rules you follow unconsciously
• Time to write, test, and refine (typically 2-4 hours for a solid set)
• Ongoing maintenance as you discover gaps

The challenge isn't the feature itself. It's that most people don't know what to put in custom instructions—because articulating your own writing patterns is surprisingly difficult.

This is exactly what Style Profile extraction solves: it analyzes your actual writing and generates the comprehensive content your custom instructions should contain.

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The 7 Dimensions Your Custom Instructions Should Capture

Your writing has structure beyond what simple rules can convey. Here are seven dimensions that comprehensive custom instructions need to document—and why most people's instructions fall short:

1. Sentence Rhythm Patterns

You don't just write "short" or "long" sentences. You have a rhythm.

Maybe you use short declarative sentences for emphasis, then longer ones to explain context. Maybe you open with a fragment for punch. Maybe your average sentence is 12 words, but you hit 25+ when building an argument.

What instructions say: "Keep sentences concise."

What your actual pattern is: "Average 10-14 words. Use 3-6 word fragments for emphasis. Longer sentences (18-25 words) acceptable when explaining trade-offs or context."

2. Punctuation Fingerprint

Em-dashes, semicolons, parentheticals, colons—these aren't random. They're part of your voice signature.

Some writers use em-dashes constantly. Others avoid them entirely. Some use parentheticals for asides (like this). Some prefer semicolons to connect related thoughts; others never touch them.

What instructions say: Nothing.

What your actual pattern is: "Heavy em-dash user—for emphasis and asides. Occasional semicolons. Rare parentheticals. Minimal exclamation points (only genuine excitement)."

3. Transition Architecture

How you move between ideas is distinctive.

Do you use explicit transitions ("However," "That said," "The result?")? Or do you rely on paragraph breaks to signal shifts? Do you use numbered sequences for processes? Bullets for options?

What instructions say: "Write clearly."

What your actual pattern is: "Use explicit transitions sparingly—prefer paragraph breaks. Never 'Moreover' or 'Furthermore.' Acceptable: 'That said,' 'The result?', 'Here's the thing.'"

4. Formality Spectrum by Context

You don't have one formality level. You have a spectrum, and you shift based on audience.

An email to your CEO reads differently than a Slack to your team. A client proposal sounds different than internal documentation.

Custom instructions typically capture one default. Style Profiles map your entire spectrum. For a complete walkthrough of writing effective custom instructions, see our complete guide to custom GPT instructions.

What instructions say: "Professional but approachable."

What your actual pattern is:

5. Opening and Closing Signatures

How you start and end communications is often distinctive—and consistently overlooked.

Do you lead with context or the bottom line? Do you use greetings or dive in? How do you sign off?

What instructions say: Nothing specific.

What your actual pattern is: "Skip greetings in internal email. Lead with the decision/ask/update. Sign off with first initial only for internal, full name for external. Never 'Best regards'—use 'Thanks' or 'Cheers' depending on relationship."

6. Hedge and Certainty Calibration

Some writers hedge constantly ("I think," "perhaps," "might be worth considering"). Others state things directly ("This is the approach," "We need to change").

Your balance between hedging and certainty is distinctive—and often context-dependent.

What instructions say: "Be direct."

What your actual pattern is: "Hedge when uncertain or inviting input ('One option would be...'). State directly when recommending ('The right approach is...'). Never hedge on facts or data. Always hedge on predictions."

7. Anti-Patterns and Forbidden Phrases

What you don't do is as distinctive as what you do.

Everyone has phrases that make them cringe, corporate-speak they refuse to use, structures they avoid. These negative rules are rarely documented in custom instructions.

What instructions say: Maybe one or two examples.

What your actual pattern is: A comprehensive list:

• Never: "Per my last email" → "Following up on..."
• Never: "Reaching out" → Just state the purpose
• Never: "Synergize" or "leverage" → "improve" or "use"
• Never: Open with "I hope this finds you well"
• Never: Bury the lead—main point in first two sentences, always

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Side-by-Side: Manual vs. Extracted Custom Instructions

Here's what the difference looks like in practice. A Style Profile is what your custom instructions would look like if you could analyze your own writing objectively:

Manual Custom Instructions (~500 words)

I'm a VP of Marketing at a B2B SaaS company. I communicate
primarily via email to executives, peers, and direct reports.

Write in a professional, direct tone. Lead with the main point.
Use short sentences. Prefer active voice. Avoid corporate jargon.

For executives: Be formal, lead with numbers.
For team: Be more casual, open to questions.

Never use exclamation points. Never say "synergize."
Keep emails under 5 sentences when possible.

This is better than nothing. It captures basics. But notice what's missing: specific sentence rhythms, punctuation patterns, transition preferences, opening/closing signatures, the full formality spectrum, hedging calibration, and comprehensive anti-patterns.

Style Profile: Comprehensive Custom Instructions (~5,000 words)

A Style Profile—generated through systematic extraction—includes everything your custom instructions should contain:

1. Writing DNA Summary (400 words)

• Formality baseline and spectrum
• Sentence rhythm analysis (with metrics)
• Punctuation fingerprint
• Directness calibration
• Vocabulary preferences

2. Pattern Documentation (1,200 words)

• Positive patterns with examples
• Anti-patterns with alternatives
• Transition architecture
• Opening/closing signatures
• Emphasis techniques

3. Context Adaptation Matrix (800 words)

• Audience-specific rules (5-10 audiences)
• Formality shifts by relationship
• Communication type adjustments (email, Slack, docs, social)

4. Multilingual Rules (if applicable, 600 words)

• Language-specific formality defaults
• Cultural adaptations
• Phrase translations that preserve voice

5. Master Prompt (2,000+ words)

• Structured instructions for AI deployment
• Example library
• Decision trees for context-dependent choices

The depth difference isn't incremental. It's structural. If you use Claude, we also have a dedicated guide on how to make Claude sound like you with platform-specific techniques.

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Before/After: Same Prompt, Different Outputs

Prompt: "Draft an email to my team about the Q2 deadline moving up by two weeks."

With Basic Custom Instructions

Subject: Q2 Deadline Update

Hi team,

I wanted to let you know that the Q2 deadline has been moved up by two weeks. This means our new target date is April 15th instead of April 29th.

Please adjust your schedules accordingly. Let me know if you have any questions or concerns about meeting the new timeline.

Best, Sarah

Analysis: Correct. Clear. Generic. Could be written by anyone with "professional, direct" instructions.

With a Style Profile

Subject: Q2 deadline moved — now April 15

Team —

Quick heads-up: Q2 deadline shifted two weeks earlier. New target is April 15.

Three things to know: • Marketing assets need final review by April 8 • Engineering freeze starts April 10 • If this creates blockers, flag them by EOD Friday

I know this is tight. Let's sync Thursday to pressure-test the timeline.

— S

Analysis: Distinctive. The em-dash greeting. The compressed subject line. The bullet structure. The specific dates and action items. The "flag by EOD Friday" language. The initial-only sign-off. These aren't random—they're documented patterns from the Style Profile.

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When Manual Custom Instructions Are Enough

If you can document all seven dimensions yourself, manual custom instructions work well. Here's when DIY is sufficient:

1. Your communication is relatively uniform

If you mostly write the same type of content to similar audiences, a focused set of instructions can cover your needs.

2. You can articulate your patterns

Some people genuinely know their voice. They can describe their punctuation habits, sentence rhythms, and audience shifts without analysis. If that's you, DIY works.

3. "Good enough" meets your threshold

If you're using AI for first drafts you'll edit anyway, baseline instructions reduce editing time without needing perfection.

4. You have time to iterate

Building effective custom instructions is a 2-4 hour project, plus ongoing refinement. If you have that time and patience, DIY pays off.

5. Budget is the constraint

Custom instructions are free. If investing in a Style Profile isn't feasible, optimized DIY instructions are absolutely the right choice.

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When You Need Systematic Extraction

Most people can't manually document all seven dimensions—they don't know their own patterns well enough. Style Profile extraction becomes necessary when:

1. You communicate across many contexts

Executives, clients, team members, public content—each requires different calibration. Documenting all your context shifts is complex.

2. Your voice has subtle, hard-to-articulate patterns

You know something's "off" in AI output but can't explain why. That gap between sensing and articulating is exactly what Style Profiles capture.

3. You're multilingual

Mapping voice across languages multiplies complexity. Your English directness, Japanese formality, and French sentence rhythm are all distinct—and all need documentation.

4. Authenticity has professional stakes

When sounding "generic" damages your personal brand, client relationships, or leadership presence, the depth difference matters.

5. You've tried DIY and hit limits

If you've optimized custom instructions but still edit 60%+ of AI output, you're experiencing the coverage gap that Style Profiles address.

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The 7-Dimension Test

Want to know whether custom instructions are sufficient for your needs? Test your current setup:

1. Sentence rhythm: Does AI match your actual sentence length variation?
2. Punctuation: Does it use em-dashes, semicolons, parentheticals the way you do?
3. Transitions: Does it move between ideas the way you naturally would?
4. Formality spectrum: Does it shift appropriately for different audiences?
5. Opening/closing: Does it start and end communications like you?
6. Hedge calibration: Does it balance certainty and hedging like you?
7. Anti-patterns: Does it avoid the phrases you'd never use?

If AI passes all seven, your custom instructions are capturing your voice well.

If it fails three or more, you're likely experiencing the coverage gap that Style Profiles address.

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The Bottom Line

Custom instructions are the container. The question is what to put in them.

You can manually write that content—if you can objectively analyze your own writing patterns across all seven dimensions. Most people can't. They write vague rules like "be concise" because they don't know their actual sentence rhythm is 10-14 words with fragments for emphasis.

Style Profiles are the comprehensive content your custom instructions should contain, extracted through systematic analysis of your actual writing. They capture what you'd document yourself if you could see your patterns objectively: sentence rhythm, punctuation fingerprint, transition architecture, formality spectrum, opening/closing signatures, hedge calibration, and comprehensive anti-patterns.

The goal is the same either way: detailed, specific custom instructions that capture all seven dimensions of your voice. The difference is methodology—manual self-analysis versus systematic extraction.

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Build Better Custom Instructions

Option 1: Audit Your Current Setup

Use our free Voice Assessment worksheet to check your custom instructions against all seven dimensions. See exactly where your current instructions fall short.

Option 2: Generate Comprehensive Instructions Automatically

Take the free questionnaire and get a Writing DNA snapshot showing what systematic extraction reveals about your voice—the patterns you'd include in your custom instructions if you could see them objectively.

Your Style Profile becomes the detailed content you paste into ChatGPT's custom instructions, Claude's system prompt, or any AI platform. Same container, 10x better content.

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Get Your Free Writing DNA Snapshot

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