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 can't help but notice that they always seem to be new accounts as well as completing tasks for the games that are notorious for rejecting every other player multiple times, after taking over a day to "Verify" screenshots; they get each task accepted in less than a hour or two. I have a strong sense of shenanigans here...