You can install Steam Guard using the following steps:

1. Install the Steam application on your mobile phone and log in to your Steam account.

2. Click on the menu in the upper left corner (≡), then select "Steam Guard" and choose "Add Authenticator."

3. Enter the phone number where you will receive the authentication key via text message. Input the key sent to that phone number and keep the recovery code.

4. Every time you log in to Steam, enter the authentication key displayed in your Steam Guard within the Steam application.

※ It's essential to keep the recovery code in case you lose your Steam account or mobile authentication device.

※ For more detailed information on Steam Guard, please refer to the customer support page provided by the Steam platform.