Setting up Steam Family Sharing is fairly simple, but first you’ll need to turn on Steam Guard, so open the Steam client, select “Steam” in the upper-left corner, then "Settings," and finally "Account." On that panel, you'll be able to set up Steam Guard, which uses the Steam mobile app to provide verification codes when you attempt to log in from an unrecognized device.

Once Steam Guard is on, log into Steam on the laptop or desktop you want to approve for Steam Family Sharing, then once again head into the “Settings” panel but this time select "Family" from the sidebar. Click the box next to "Authorize Library Sharing on this computer" and select the local users with whom you want to share your library.

Libraries can only be used by one person at a time, and the account holder always has priority access to their games. Make sure you only share your library with trustworthy friends, though. If someone cheats in a game protected by Valve Anti-Cheat while playing your library, your Family Sharing privileges could be revoked, and your account banned.