Enable Steam Guard on both accounts
Log in to the Steam client on the computer you want to share from
Go to Steam > Settings > Family
Turn on Family Library Sharing
Authorize the computer and select the accounts allowed to use your library
Log out of the owner account
Log in to the other Steam account on the same computer
Download and play eligible shared games from the library
Keep in mind only one person can use the shared library at a time
Keep in mind not all games are shareable
Keep in mind DLC is only shared if the borrower owns the base game
Keep in mind VAC-banned accounts cannot use Family Sharing
Keep in mind some region-locked or subscription-based games may not be shared