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.
When I am trying to get some soul gems they would have me do really crazy things just to get a small amount of soul gems. Its really annoying but all I am saying is that they should add easier things to do like watch a video or something like that.