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.
How much are you attempting to earn in a specific amount of time? And what ways are you using to reach that goal? I have a daily goal for 100 SG, earned mainly by Gamehag Katana. Only requires one and a half hour and I can do that while I take my breaks at work or when sitting in queue in a Moba, or while waiting for another game to load.