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.
Bomber Barn itself is free, but Alienware Arena is offering free DLC for it right now; currently ~13k keys left. Grab one if you're interested.