5 pastry bags (16-inch or 18-inch bags are preferable but 12-inch bags would work too) or resealable plastic bags; piping tips (large star tip #825, small French tip #32, plain tip #6, small star tip #30); interchangeable collars or couplers; fondant smoother; small paintbrush; wooden skewers; toothpicks; fondant rolling pin or wooden rolling pin; rotating cake stand/turntable; large paintbrush (for dusting)
Separate the buttercream into 4 separate bowls (2 larger bowls and 2 smaller bowls). In one of the smaller bowls, use leaf green, chocolate brown, yellow and black gel colors to make a dark, evergreen color. In one of the larger bowls, use a few drops of yellow and chocolate brown gel colors to make a mocha color (lighter than the shade of brown fondant you have). In the other larger bowl, use chocolate brown and black gel colors to make a dark brown (darker than the shade of brown fondant you have). In the other smaller bowl, use red gel color to make a Christmas red color.
Sprinkle your clean table with cornstarch. Knead your light brown/mocha fondant (similar to bread dough) until it is smooth. Using a rolling pin, roll the fondant so that it is approximately a 14-inch circle that's 1/4-to-1/2-inch thick. Sprinkle with cornstarch and roll onto the rolling pin. Unroll onto the top of the cake and smooth out the top and sides with a fondant smoother so that there are no air bubbles.
Roll a thin snake of light brown/mocha fondant for the border of the cake.
Make antlers: Roll modeling chocolate out to about 1 1/2 inches thick. Freehand cut 2 antler shapes. Slide a wooden skewer into each antler as far as it will go. Paint with edible gold paint.
Make ears: Roll the leftover light brown/mocha fondant into 2 teardrop shapes. Use your thumb to indent the centers of each to create the ear shape. Place a toothpick or wooden skewer in the bottom of each one. Paint the center of each ear with edible gold paint.
Take a small amount of black fondant and roll out a snake for each eye. Use smaller pieces to create the eyelashes. Accent eyelashes with edible gold paint.
Create the mane by piping alternating rosettes down the center of the cake with the mocha and dark brown buttercream. Add smaller green shells and top each of those with a red holly berry bunch.
Pipe a red nose below the eyes using a large plain tip.
Add the antlers and the ears to the top of the cake.