1. First, you need to install the Steam app and log into your account. You can download the app here.
2. Sign in with your Steam account
3. Select "Steam Guard" and enter your phone number.
4. Steam will send you a confirmation email. Click on the "Add Phone Number" in the received email. Then return to the Steam app and click "Next".
5. Steam will send you an SMS shortly after that with the confirmation code.
6. Enter the code to confirm that you can get messages from Steam. If you didn't get the code, click "Resend" and try again.
7. You will see the Recovery code When the code is successfully entered. Write down the code and keep it in a safe place.
8. When the mobile authenticator is enabled, you'll see a unique Steam Guard code that updates periodically. You'll be asked to enter the code when you log in to Steam.