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.
The Steam Chest is pretty much the best deal. The worst items are still very good. You have 1/2 change getting 1/2 of chest price back and 1/2 chance of getting good game. Other chests have bigger chance of game, but you can get rune with value of 10 souls and then you will be very very sad