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 got turned down on my attemt to gain the bonus souldgems when WarThunder was on daily special. I spent a lot of time on it..turns out it was for nothing. I think I should warn others.