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.
Based on my experience after using this site for almost 1 year, chances of getting rune are always higher than getting games.
Runes> random games> AAA games
I had gotten FIFA 17 from a chest before. It was a chest from a redeemable code.