This tutorial assumes you want to set up a simple, two-signer, equally-weighted multi-sig account. If you’d like to accomplish something more complicated, learn more in the Stellar Dev Docs.
Create two accounts, preferably on a device not connected to the internet. If you already have an account and want to add a signer account to it, just create one new account. You can create accounts using the Stellar Account Viewer (online) or a paper wallet generator (offline). Copy down these two accounts’ secret keys and store them in multiple safe locations.
Visit the Stellar Laboratory (the official Stellar transaction builder). Enter your primary public key in the “Source Account” text field. A blue button will appear after you type in your public key that says “Fetch next sequence number for account starting with…”. Click it to populate the text field with your next transaction number.
Skip down to the “Operation Type” section and select "Set Options".
For Master Weight, input “1”.
Skip “Low Threshold”.
For “Medium Threshold”, input “2”.
For “High Threshold”, input “2”.
Skip the rest of the inputs and click the button at the bottom left of the page that says “Sign in Transaction Signer”.
After signing the transaction, scroll down and click the 'Submit to Post Transaction endpoint'.
If you’d like to verify, visit the Endpoint Explorer (make sure you are on the public network). Select "Accounts" and then "Single Account". Paste your primary account ID in the text box next to "Account ID" and submit.
In the same JSON, find "Signers" and confirm that the proper account IDs, weights, and types are present. If everything is correct, your multi-sig setup is ready!