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 think in gamehag you should see how much sg percantage you accomplished to open a spesific chest so we can plan doing which tasks and when to doing them etc.