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.
After each game game chooses random players and drops random items. I saw graffiti, skins, cases, but never saw someone getting a key for a case. Is that even possible to get one ?