To enable Family options, simply head into the Settings tab of Steam and enable it for your own account or any Family account associated with yours. You can then enable Family View, which will alter what content your own account or Family accounts can access and see unless they input a special code.
Managing which games are in your Family Library is fairly straightforward: Boot up Steam, go into your library, right-click the game you want to share and then click "Add to / Remove from Family Games." These are the games your Family accounts will then be able to access.
There's a Steam Family Options group on Steam you can join if you're looking for more info or need to troubleshoot. In order to start using Steam Family Options, you'll need to update your Steam client.
Update: The original text in this article mentioned Steam Family Sharing, which is entirely separate from Steam Family Options. We apologize for the error and have corrected the article.