Bake and cool cake. Position cake on foil-wrapped cake board. Ice sides of cake with white buttercream. Ice top of cake with leaf green buttercream. Use tip 8 to pipe bead bottom border with leaf green buttercream.
Using largest star cutter from the Christmas Cookie Tree Kit and black FoodWriter, trace star outline on the back side of Bright Yellow Sugar Sheets! Cut star out with scissors. Use tip 3 to pipe piping gel along edge of star and sprinkle with yellow sugar. Attach star to cake.
For numbers, place Black Sugar Sheets! on foam. Using the numbers 2 and 1 cutters from set, cut out numbers. Brush backs with piping gel and attach to star.
Using punch with Swirls Cutting Insert, cut swirls from Sugar Sheets! colors. Place on cake.
For message, use letters from Alphabet Sugar Sheets! and place on cake. Brush back of letters with piping gel if needed.