Note: you cannot deploy a "legacy" wallet as multisig. Also if you have an existing already deployed account, you would need to create a new account to deploy it as multisig.
Create a new wallet (or account if you already have a wallet) and deposit some TON to the account
Click on Deploy
3. Select Multi-signature wallet
4. Select the amount of custodians and enter the public key for each custodian
5. Click on next and confirm the deployment of the wallet
This is it, your multisig wallet has been deployed!
If you then need to send a transaction, each of the custodian will need to confirm it.