Pretty simple, as per most other Steam giveaways - register for the site, add to your cart, checkout - steam code goes into "My Games"
Relic Hunters Zero is a game about shooting evil space ducks and evil space turtles with tiny cute guns. It’s fast, it’s tactical, and feels delicious and smooth to play. Unlock new weapons, characters and relics to defeat the Ducan Commander.
This "Steam-key" version of the game contains Steam Trading Cards.
Direct Steam Link:
http://store.steampowered.com/app/382490/
Link to get the steam key on the right which gets you trading cards as well ->>>>>
Clarification:
If you don't redeem the game using a steam code, you don't get trading cards. If you redeem it using a steam key, it's considered a game purchase even though you havent paid for the key and therefore get the trading cards. If you just want the free game and don't care about the cards - click the link directly in steam for the game, but its then on a f2p model and you dont seem to get trading cards unless reaching minimum spend threshold on purchases (around $10).
Again; you can get the game for free on Steam etc. Generating the code gets you the trading cards and the game in one fell swoop.
Local co-op. Awesome!