Play games
Choose the game that interests you and play itfor free.
Complete the tasks
Every game has some tasks 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 really like this game... but if you are looking just for free gems I can not recommend this to you. Second and third quest takes days, maybe weeks.