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.
Hallooween sale on HB, get this game for free
https://www.humblebundle.com/store/search?sort=discount&filter=onsale
also there's huge sale on steam till 1st Nov