You can get Serious DLC for The Talos Principle just by clicking on the button Free when you open the giveaway link on SteamDB.
This will open your desktop Steam client and add the DLC to your library if you also own the base game.
About this Content: The new Serious DLC replaces the godlike voice of Elohim with completely NEW, rewritten and humorous Serious Sam script.
Serious Sam voice has been recorded by longtime Serious Sam voice actor John J. Dick.
It also includes a new Serious Sam player model for use in The Talos Principle.
Note: This content requires the base game The Talos Principle on Steam in order to play.