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.
It took a few weeks to reach level 200, but a at least 3-4 months to reach level 300. Quite a hard quest to achieve.