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 was thinking about buying the 2nd Mafia and 3rd Mafia, but I am not sure if the code for the 3rd one is a steam code or not. Anyone knows?