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 did this offer but it seems to be rather fast at the beginning but slows down incredibly later. It slows down incredibly. Furthermore, one event leads to another, which requires a chain of upgrades which further create the illusion it is achievable. It is fully possibly to be finished in 20 days but it requires absolute planning. At the very beginning, try to get as far as possible and leave the building projects overnight to finish. This helps decrease the time wasted. I got level 16 in 20 days just because I wasted time initially. It seems I get nothing.