This is the EASIEST way to get the badge and only requires 1 purchase!
Shopping Trip – Log onto Steam using the mobile app that is available for iOS and Android!
It Pays for Itself – Purchase a game that is discounted. Basically, purchase any game!
The Holiday Spirit – Purchase a game as a GIFT. Choose to put it into your Steam Inventory. You will then have the choice to give it to someone, or you can claim it for yourself!
Checking it twice – Have 10+ items in your Steam Wishlist. I automatically unlocked it since I had 30 items in my wishlist.
After you get these, you have got the Steam Holiday Sale 2012 badge and you have the choice to not do anything more.
If you are a completion-ist, vote on the “Community Choice” game every 8 hrs. You get a achievement for voting 5 times (Be The Decider) and again at 15 times. (The Will of the Electorate)
Hope this guide has helped you!