Enable Steam Guard on your account
Log in to your Steam account on the friend’s computer
Open Steam Settings
Go to Family or Family Sharing
Authorize the computer for Family Library Sharing
Select the friend’s Steam account to allow access
Log out of your account
Have your friend log into their own Steam account
Open the shared library and install an available game
Keep in mind only one person can play a shared library at a time
Note that some games may not be shareable
Note that DLC and in-game purchases may not always be shared
Remove access later from the Family Sharing settings if needed