Enable Steam Guard on both accounts
Open Steam on the sharing account
Go to Steam > Settings > Family
Turn on Family Library Sharing
Authorize the device and select the accounts to share with
Log out of the sharing account
Log into the other account on the same computer
Download and play eligible shared games from the library
Note that only one person can play a shared library at a time
Note that not all games support sharing
Note that DLC and some subscriptions may not be shared