You play David, an awfully small abstracted chosen hero tossed into a blissfully simple world overrun by giant culminations of evil physics-beasts. As David, you are given a slingshot-esque gift that you will have to use to conquer your colossus foes. Be there AND be square.
Open the Steam client, login and locate the Games tab on the top menu. Click this and then select “Activate a Product on Steam…”
Enter your unique code that's displayed on the 'Orders' page of CDKeys.com and follow the prompts to activate.
Your game is now viewable in the “Library” tab and is available to download/install when you’re ready. You can find additional support for the Steam client here: https://help.steampowered.com/en/wizard/HelpWithSteam