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.
The quickest method i found was to go to the very top of the map where there is the dirt fields. I started farming the little monsters and i was able to get level 9 after about 20 minutes. hope this helped and goodluck!