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.
According to FAQ:
"May my Soul Gems vanish?
Soul Gems are not bananas, they can't rot. If you can't believe, check it out in a geological museum! :)"
Yet coming back to this site (I guess for a very short time in such case and ready to loudly badmouth it after leaving again) according to my notification I've been stripped of 1700 Soul Gems because:
"Due to the lack of your activity on the website, you’ve been turned into a Toad, you’ll have to spend 50 Soul Gems every 7 days." ("have to spend" meaning, that 50 gems were being removed from my account every week for nothing, and, yes, they still have been removed yesterday, even after I've received a login reward 2 days ago - https://i.imgur.com/PqzPAuF.jpg)
Which actually is worth peanuts, but still feels like being cheated out and is quite upsetting, especially in light of FAQ's statement suggesting that the Soul Gems do not vanish :((.