For Android and iOS devices

Setup is done directly from your mobile device.

Or you can send this link to your mobile device:
http://itunes.apple.com/us/app/steam-mobile/id495369748

Or you can send this link to your mobile device:
https://play.google.com/store/apps/detailsid=com.valvesoftware.android.steam.community
The Android and iOS apps will follow the same setup flow. For the purposes of this walkthrough, the images below show the iOS app.
Once you will installed the app and signed in with your Steam account credentials…
Tap the menu icon at the top left to expose the app’s main menu. Select “Steam Guard” from the top of the menu.

Here you can view your current Steam Guard settings. Tap “Settings” to make changes.

Tap “Get Steam Guard codes on my phone” to make your mobile device an authenticator.

Follow the steps to confirm your mobile device.

Enter your phone number. Add the international prefix from the drop down.

Steam will send you a text message with a confirmation code.

Enter the code to confirm you can receive SMS messages from Steam. If you didn’t receive the code, tap “Resend code” to try again.

When the code is successfully entered, we will present you with a Recovery code. Write down the code and store it in a safe place. Do not skip this step.

When the mobile authenticator is successfully enabled, you will be presented with a unique Steam Guard code that will periodically refresh. When you log in to Steam you’ll be asked to enter the code.
