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.
Not sure if it's a new thing, but I discovered that the EXP gain from commenting in articles is a bit more than in the forums. I'm currently at level 2 and a comment in the articles gave me around 4-5% of EXP while in the forums I only got around 2%. The number might be different for other levels though.