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 am ashamed to admit that I played this for some time lol. Although to be fair it turned out to be the better one among its carbon copy cousins on the market.