This is simple enough! Just /dance in dalaran with a snowman as a snowman! You can get your kit by completing the Stolen Winter Veil Treats quest-line. Kaymard Copperpinch in the Valley of Strength, Orgrimmar and Macey Jinglepocket in the Commmons, Ironforge are the questgivers. Here's the steps involved:
You're a Mean One...: The Stolen Treats are looted from The Abominable Greench that spawns around the mountains. He yells "Rawwr!" whenever he appears. The Greench has many different spawn points, so check out this map for a better idea of his locations.
24 Hours after completion of this quest you will receive your Winter Veil Disguise Kit in the mail! This will also earn you the Simply Abominable achievement. Happy dancing!