Preheat oven to 325°F. Spray Ugly Sweater Cake Pan with vegetable pan spray.
In large bowl, beat cake mix, eggnog, oil, eggs and nutmeg with electric mixer at low speed 30 seconds. Scrape bottom and sides of bowl; beat at medium speed 2 minutes. Pour batter into prepared pan, spreading to even thickness.
Bake 39-42 minutes or until toothpick inserted into center of cake comes out clean. Cool in pan on cooling grid 10 minutes. Remove from pan; cool completely on grid.
To decorate: With spatula, ice cooled cake. Using tip 233 and green icing, pipe tree on cake. Using tip 46 and red icing, pipe outline neckline, sleeve cuffs and bottom sweater band. Attach sprinkles.