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.
29F6A-5#M35-BY8X8 ;
Just replace the # with the missing letter.
Finally the not-so-cheap one :)
N.B. I close the threads each time just to prevent the possible flood, so sorry if I don't leave you the chance to comment if you take the key.