[Installing Steam Mobile Authenticator]
1) Install Steam application on your mobile phone:
2) Wait until the application is completely installed
3) Select Steam Guard from the upper left menu of the application
4) Select the Add Authenticator button
5) Enter the number where you'd like to receive the authentication key
6) Enter the authentication key into the activated application screen
7) Check the recovery code. This recovery code is valuable information that is essential when trying to recover your lost Steam account or your Mobile authenticator.
8) Once the setting is completed, each time you log in to Steam, an authentication key will be sent to your registered mobile device.