3 Ways to Build an AI Second Brain

A second brain is one place where all your important information lives. Your files, your documents, your projects, your notes. The point is that AI can reach that place, so you stop re-explaining who you are and what you sell every single time you open a chat.

Three ways to build one, counted down. Any of them beats none of them by a wide margin.

Read this before you build anything

Every one of these fails the same way, and it is not technical.

People build the structure and never put anything in it. Beautiful folder trees, careful naming systems, tags, plugins, colour coding, and no content. A perfect empty filing cabinet is worth exactly as much as no filing cabinet at all.

So before you read the methods, take the rule that makes any of them work.

Put five real files in on day one. Not a folder system. Not tags. Five files with actual information about you and your work.

If you only do that and nothing else on this page, you are ahead of most people using AI daily.

At a glance

Number three: Claude Projects, the easiest

A Project is a container inside Claude that holds files and instructions and remembers them between conversations. Fifteen minutes, no technical skill needed.

Step 1: Create the project

Open the sidebar, click Projects, then New project. Name it after what it is for. "My business" is fine. Do not spend time here.

Step 2: Upload the right things, not everything

The instinct is to dump your whole drive in. Resist it. More files is not more useful, and a project stuffed with three years of miscellaneous documents produces vaguer answers than one holding six good ones.

Three kinds of thing, in this order:

• A one-page document about you. Your role, what the business does, what you are trying to achieve, what you are responsible for. Write it as though briefing somebody sharp who starts Monday.
• The documents you reach for constantly. Price list, brand guidelines, client list, standard terms, the proposal template you keep adapting.
• Three examples of your best work. Actual finished things. The email that got replied to, the proposal that closed, the description you were happy with.

That last group carries more weight than people expect. Three examples of good work teach it your standard better than thirty documents describing your standard. Telling it you write plainly does less than showing it two paragraphs where you did.

Step 3: Write the project instructions

This is the field most people leave blank, and it is the one that changes the output most.

Cover four things:

1. Who you are. One or two sentences. Role, business, who you serve.
2. What you want it to do. Draft things in your voice, or check your thinking, or both. Be specific about the job.
3. How you want it to talk to you. Short and direct, or thorough. Ask before assuming, or make a call and flag it. This one is worth being fussy about, because it decides whether using it feels good.
4. Which files matter most. If the price list is the thing it must never get wrong, say so.

Write it once, adjust it after a week of real use, then leave it alone.

Step 4: Keep it current, or it rots

Once a month, delete what is out of date and re-upload what changed.

This sounds like housekeeping and it is not. An old price list is worse than no price list, because it will quote from it confidently and you will not notice until a customer does. Same for a client list with people who left, or terms you no longer offer. Stale context does not degrade gracefully. It produces wrong answers in a completely convincing tone.

Put it on the calendar. Ten minutes.

The catch worth knowing

Two of them, and you should know both before you commit.

It chooses what to read. Rather than reading every file every time, it pulls what it judges relevant to your question. Usually right. But a detail buried in your ninth document may simply not surface, and you will not be told it was skipped. The fix is keeping the number of files small and the important things near the top of them.

It lives inside Claude. Everything you build here works in one place. Move to a different AI in a year and you are re-uploading by hand and rewriting the instructions. Not fatal, but worth knowing you are choosing convenience now over portability later.

Number two: Obsidian

Obsidian is a free note-taking app where your notes are plain files sitting on your own computer. Nobody else holds them. About an hour to set up, plus a few weeks before it feels natural.

Step 1: Install it and make a vault

Get it from obsidian.md. A vault is just a folder. Put it somewhere backed up: your Documents folder, or a synced drive.

Step 2: The smallest folder structure that works

Four folders. Not fourteen.

inbox/
notes/
projects/
reference/

• inbox for quick captures you have not sorted
• notes for finished thinking
• projects for what is active, one file per project
• reference for material from elsewhere

Name files in lowercase with dashes. brand-voice, not Brand Voice FINAL v2.

Step 3: Connect AI to it

Three options, easiest first:

• Point a coding assistant at the vault folder. Claude Code or Codex. Despite the name, you are not coding. You are pointing it at a folder of notes and asking questions. This is the easiest path and the one most people end up on.
• Use an AI plugin inside Obsidian. Keeps everything in one window. More setup, and you are dependent on whoever maintains the plugin.
• Drag files into Claude by hand. Crude, works today, fine while you decide whether you are going to stick with this.

The warning that matters most

This is the section to actually read.

People spend their first month installing plugins and designing systems instead of writing anything down. There is a plugin for everything, a graph view that looks impressive, a hundred videos about the perfect setup. It all feels like progress. None of it is.

You can tell it is happening when you have spent four evenings on Obsidian and cannot point at a single note containing information you did not already have.

Install one or two plugins at most in your first month. Zero is genuinely fine.

The app does the job unmodified. Every hour spent configuring it is an hour not spent filling it, and the filling is the entire point.

Number one: a folder of files

No app at all. Just a folder. Thirty minutes, and it is the one worth using.

Step 1: Make the folder

On your desktop, a folder called brain, with three inside it:

brain/
  about-me/
  projects/
  reference/

That is the whole structure. Do not add more yet.

Step 2: Write the one file that matters most

In about-me, a file called context.md with six sections:

1. Who I am
2. What I am working toward (with actual numbers)
3. How I work
4. How I want AI to talk to me
5. What I keep having to explain
6. What is in this folder

Section five is the one that pays for the whole exercise. Anything you have typed into a chat more than twice belongs there permanently. What your business does. Why you price the way you do. The thing every new client misunderstands. Write it once, properly, and never type it again.

Section two is worth being specific about too. "Grow the business" is useless. "Get to twelve retainer clients by December, average deal above three thousand" gives it something to actually reason against.

Step 3: Use markdown files, named like a human

Plain .md files, lowercase with dashes. brand-voice.md. pricing-logic.md. who-we-serve.md.

Markdown is just text with a few formatting marks. A # makes a heading, a - makes a bullet, and asterisks make things bold. That is most of it. Any app can open these, and they will still open in ten years, which is not true of most places you could put this.

The naming matters more than it looks. You are writing filenames for something that decides what to read based on the name. pricing-logic.md gets found. Untitled 3.md does not.

Step 4: Point Claude Code or Codex at it

Aim a coding assistant at the folder and ask questions normally.

Do not let the word "code" put you off. You are not programming. You are giving a tool permission to read a folder, and then talking to it in English. It reads the files and answers using your actual information rather than guessing at a generic version of your business.

Ask it something you would normally have to explain first, like drafting a reply to a client asking for a discount. If it comes back sounding like you and referencing your real pricing, it is working.

The one con

There is no interface. It is a folder.

That feels bare for roughly two days and then stops mattering entirely, because you were never going to sit and admire it. The AI was the one reading it. What you lose in polish you get back in the fact that nothing here can be discontinued, repriced, or locked to one company.

Decide

• Not sure, or starting today: Claude Projects. No penalty. The files come with you later.
• Already a note-taker: Obsidian. You will actually use it, which beats theoretically better.
• Want the most power and do not mind plain files: the folder.

Pick in ten seconds

Do not spend a week choosing. The gap between any of these and nothing is enormous. The gap between them is small.

Pick one, then go write those five files. Suggested starting five:

1. Who you are and what the business does
2. What you sell and what it costs
3. Who your customers are and what they worry about
4. How you write, with two examples of you sounding like yourself
5. The thing you explain to every new client, written out once

That is one hour. It is worth more than any amount of structure, and everything you build afterward is better because it exists.