This guide will give you step-by-step instructions on how to enable Steam Guard.
Steam Guard is a security feature which is designed to put an additional layer between your account and unauthorized parties. The official Steam Support documentation for Steam Guard can be found here.
Features that Require Steam Guard
Certain features on Steam require Steam Guard to be active for 15 days before they can be used. If you disable Steam Guard for any reason you'll be unable to use them. Even if you disable Steam Guard momentarily and re enable it, you'll still have to wait the standard 15 days before you can use these features again.
Each new device or browser authenticated to your Steam account will need to be protected by Steam Guard for at least 15 days before using any features requiring Steam Guard. New devices authenticated within your household and previously authenticated devices and browsers will not be restricted.
With Steam Guard enabled, whenever you login to Steam through the client, browser or phone app, you'll receive a notification requesting a Steam Guard code. The code will only be requested whenever a new device or browser attempts to login.
The notification will appear after you type in your Steam username and password.
The e-mail will be sent from email@example.com to your verified contact address on Steam. It will include a code which must be typed into the box before you'll be able to proceed with logging in.
You're also able to set a friendly name for the device or browser you're logging in with. This is a way to distinguish the device or browser, should Steam ask you to verify login attempts in the future.
Enabling Steam Guard
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 button.
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.
If you're having problems with Steam Guard or logging into your Steam account, read the official FAQ and then contact Steam Support. Nobody else is able to assist you with these types of account related issues.