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 really miss those MarvelousGA and DupedOrNot giveaways when i always win rune. I at least won rune so i could sell it and get 50SG. That was something. But now there are no giveaways, no more codes, no more option for selling runes or games :(