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 want to using Prepaid Visa/Mastercard to buy game console online but from my personal experiences most of the online shopping website did not accept it, i tried on Amazon and Steam before, Amazon i even got banned.....So i am quite jealous to US and European user because they can buy Amazon Gift Card as a reward here while Asian user like me cannot :(