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 have this key for the game in the title. Steam link:
https://store.steampowered.com/app/355100/Dungeon_Crawlers_HD/
Here's the key: TZECR-IQBRT-48N3B
Enjoy :)