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 have heard a lot about guild wars 2 but never got around to try it. This game is very interesting. I thought some of the missions in the beginning were pretty annoying and unnecessary like collecting Raven Eggs and putting them back in the nest. Other than that its a pretty good game.