3 Ways To Build
An AI Second Brain

Step 1

Create the project

In Claude, open the sidebar, click Projects, then New project. Name it after what it is for: My Business, Client Work, Job Search. "Second Brain" is a bad name, because it tells you nothing about what belongs in it. Make one project per big area of your life, not one giant project for everything.

Step 2

Upload the right things, not everything

Click Add content and upload into the project knowledge, in this order:

1. A one-page document about you. Your role, what you are responsible for, your goals this year, who you work with, what you want help with. This one file does more than the next ten combined.
2. Your recurring documents. Brand guide, pricing sheet, client list, product specs. Anything you have pasted into a chat more than twice.
3. Three examples of your best work. Real ones, so it learns your standard instead of guessing.

Do not upload your whole hard drive. The knowledge box is generous, but not infinite.

Step 3

Write the project instructions

Almost everyone skips this, and it is what makes the project feel smart. Write four things in the instructions box:

1. Who I am. Role, business, situation, in a sentence or two.
2. What I want you to do here. Be specific: write emails to my list, review my ad copy.
3. How to talk to me. No corporate language, short answers, push back when I am wrong.
4. What to check first. Name your key files, like always check the brand guide before writing anything customer facing. That one line lifts answer quality more than anything else here.

Step 4

Keep it current, or it rots

A stale second brain is worse than none, because you trust it and it is wrong. Two habits: at the end of a good chat, have it write a short summary file of what you decided and upload that back in. Once a month, delete anything out of date and re-upload what changed. Ten minutes, on your calendar, or it will not happen.

Step 1

Install it and make a vault

Download Obsidian from obsidian.md. On first open it asks you to create a vault, which is just a folder on your computer. Put it somewhere you will find it, like Documents. If you use iCloud, Dropbox, or Google Drive, put the vault inside that synced folder so it backs up for free. Make one vault, not three, because the AI sees one folder at a time.

Step 2

The smallest folder structure that works

Four folders is enough for years:

1. inbox for anything you capture fast and sort later.
2. notes for finished thinking, which is what you want the AI reading.
3. projects for one file per active project.
4. reference for things you did not write: saved articles, docs, transcripts.

No tags or naming conventions yet. Write notes for two weeks, then let the structure follow what you actually have.

Step 3

Connect AI to it

Your vault is a normal folder of markdown files, so you have three options:

1. Easiest: point Claude Code or Codex at the vault folder. It reads across every note, writes new ones in, and answers from your own material. No plugin needed.
2. Also fine: use one AI plugin inside Obsidian if you want the chat window in the app.
3. Manual: drag the notes you care about into a Claude Project.

Step 1

Make the folder

Right-click your desktop and make a new folder. Call it brain. Inside it, make three folders: about-me, projects, and reference. Add more later if you need them, not now.

Step 2

Write the one file that matters most

In about-me, make a file called context.md. Every AI you point at this folder reads it first. Write six things in plain sentences:

1. Who I am. Name, what you do, what your job or business actually is.
2. What I am working toward. Goals for the year, with numbers if you have them.
3. How I work. Schedule, tools, what you do yourself and what you hand off.
4. How I want AI to talk to me. Direct, no filler, push back when I am wrong.
5. What I keep having to explain. Every piece of background you are tired of typing.
6. What is in this folder. One line per folder, so the AI knows where to look.

Fastest way: turn on voice dictation, talk for two minutes about all six, and tell the AI to clean it into a markdown file. Talking beats staring at an empty page.

Step 3

Use markdown files, named like a human

Every file is a .md file, which is just a text file. Name files so you know what is inside without opening them: brand-voice.md, 2026-goals.md, client-acme-notes.md. Not notes2.md or final-final.md. Lowercase with dashes, one file per topic.

Step 4

Point Claude Code or Codex at it

Start Claude Code or Codex in that folder, then ask something you would normally have to explain first, like based on everything in this folder, what should I focus on this week? It goes both ways too: end good sessions with save what we decided as a new file in projects, and the second brain grows while you use it.