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 try to do a lot of surveys, almost my main source of SGs, because I don't have any doable tasks left in games, and I believe that there should be more surveys present for us to qualify for. Even if it pays you less it could cover up for our daily task.