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.
From what I can tell, it's very difficult to talk to staff here. It would be nice if there were more specific guidelines for the kind of photos you want to complete quests. Your time would be spent better and so would ours.