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 see a lot of people posting on this community thread that this game is dead. that is not true. it is pretty much alive, a lot of people play it, it is the most played game on steam, #1 now, you can check on steam charts, it get updated almost every week, and they had some REALLY big updates in past 1 year, they made it free for everyone, added battle royale mod, made new skins...