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.
The game is still free (having the 100% discount) for the next 24 hours.
N.B. Just for personal safety I won't post the direct link, but you can easily find the game in Steam. Hurry up to extend your library! ;)