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.
Obviously WGNA has not learned their lesson. Previous thread on this exact topic is here. As you can see in the screenshot below from today (and this also applies to earlier bundles in the advent calendar), there is no listed expiration date on the missions that can be seen before the package is purchased. This would lead most people to believe that the missions do not expire, however they do in fact expire. You can find this by first winning in the tank for your daily double, then going to the missions tab, selecting the mission and looking in the upper left corner of the box to see the date listed. Same issue as last time-- expiration dates MUST BE MADE CLEAR AT THE TIME OF SALE, or you have bait and switch on your hands... Again.