Play games
Choose the game that interests you and play itfor free.
Complete the quests
Every game has some quests for which you will receive Soul Gems.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
I'm wondering how it detects whether or not you have downloaded and played the game. I just downloaded Crossout and it hasn't downloaded yet, but I was just wondering how it detects it since it doesn't make you download any application to your computer to confirm you've downloaded/played the game.