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 finally understand how to complete the first task and get accepted, by taking a screenshot of 16 victories. But what are the other tasks so I can prepare?