Open Steam and sign in to the account that owns the games
Click Steam in the top-left corner
Select Settings
Go to the Family tab
Turn on Library Sharing
Authorize the computer you want to share on
Log in to the other Steam account on the same computer
Accept the shared library access if prompted
Repeat for each account you want to allow
Make sure Steam Guard is enabled on the owner account
Note that some games cannot be shared
Note that only one person can use the shared library at a time