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 personally think that it would be more worth to spend time on updating and balancing the main game (LoL) or at least speed up the development of other side games. What do you guys think?