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 don't know if it's the same for the other games, but for the katana game, it says "For every 1000 points you get SG!" Well, I've just scored 3400, but was only paid one. It seems that minigames only give you 1 SG after reaching the goal once. After that, a page refresh is required to earn yet another SG. So, other than trying to score a personal best, it's pointless to continue a game once you've received one SG. Is this how it's supposed to work?