Enabling Steam Guard is simple. First open the Steam Client and click on the Steam
button located on the menu at the top left of the client. Once you have the menu open click on Settings
If you don't have Steam Guard enabled, it means you haven't verified your contact e-mail address with Steam, or you manually disabled it. Verifying your e-mail address will turn Steam Guard on automatically.
From the Settings page click on the Verify Email Address
Follow the prompts and click on the link provided by Steam to the e-mail address to finalize the verification process.
Once the address is verified your settings page will look similar to the screenshot below. Notice how it says Steam Guard is not enabled.
The only thing left to is to restart the Steam Client. Once the client is done restarting Steam Guard will be fully enabled. If you return to the Settings
page the shield icon will now be green and it will provide you with a date you turned the service on.
If Steam Guard did not turn on after you verified your email and restarted your client, you can click on the Manage Steam Guard Account Security
button and check the Protect my account with Steam Guard
box from the Steam Settings page. Once you click next
Steam Guard will be enabled.
At any point you can go back to the Steam Settings
page to Deauthorize access to all devices and browsers which were previously authenticated. To do this click on the Manage Steam Guard Account Security
button. From this screen check the box which says Deauthorize all other computers now
and then click next.
This will force you to re-enter your Steam Guard code next time you log in. This function is useful if you were visiting a public internet cafe, or using somebody else's computer.