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've been getting a couple of quests rejected because they said I didn't register through their link. I'm assuming it's because I didn't register then and there the moment I got there instead I went and downloaded the game then registered something like that.