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.
http://steamcommunity.com/games/572430/announcements/detail/1596955106446944465 ... I guess you have to sign up for the newsletter thingy to get a download link. I already got a direct download link via mail because I signed up for that newsletter in the past.