You can just tell them the key. Then they can go to "Activate a Product on Steam" under the Games menu and type in the key. Or, you can look in your Steam Inventory ("Manage Gifts and Guest Passes", Games menu, or under your profile page). If you have a item stand-in for the game, you can trade that to someone who can then use it to add the game to their library.
---
The Master said: "It is all in vain! I have never yet seen a man who can perceive his own faults and bring the charge home against himself."
>Analects: Book V, Chaper XXVI
---
The Master said: "It is all in vain! I have never yet seen a man who can perceive his own faults and bring the charge home against himself."
>Analects: Book V, Chaper XXVI