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 was at 50% on lvl 2 and i post useful answers and i help people, but now i am on 33% and i lose xp every day with no reason.