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.
This guide details all the cool things you can get plus shows how the system works. Haven't looked into the latter yet, but I expect it to be like last year so it'll be a nice review for older players or intro for those who missed it.