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.
Since it requires one to be level 3 to participate,I suppose it can generate certain reward.I haven't seen anyone mentioned it as a way to earn SG,so I am here to check.