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 believe you should put an additional rule to the proposed games that tasks will be generated. The game should be free, so everybody can have access to that game. There is game called Enlisted maybe add it as option?