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 tried three times already, after the 2nd try I created a ticket complaining about this, then they tell me that it's solved but after that my third try got rejected as well.. What??