Bake cake in a 9x13 cake pan and allow it to cool fully.
While the cake is baking and cooling, prepare the frosting and color half in a light green shade and half in a darker green shade.
Cut the sheet cake to make 2 large triangles and 2 small rectangles. It would be helpful to use a toothpick to mark the top first before using a knife to make final cuts.
Place one triangle cake on a cake board.
Fill a piping bag with dark green icing and attach a French tip (Wilton Tip 4B).
Pipe little stars on top of the cake until fully covered.
Place the other triangle cake on top of the icing and press gently to secure properly.
Fill a piping bag with light green icing attach a large star tip (Wilton Tip 1M) and pipe large roses on top of the cake randomly until covered. Leave the little gaps between each flower empty.
Fill the gaps by piping little stars, using the French tip and dark green icing.
Make the tree trunk now by placing one rectangle cake below the prepared triangle layer cake.
Fill a piping bag with chocolate frosting and attach a French tip (Wilton Tip 4B).
Pipe little stars on top of the cake until fully covered.
Place the other rectangle cake on top of the icing and press gently to secure properly.
Pipe little stars on top again until fully covered.
Toss some red sprinkles and mini m&m candies on the top green portion of the cake.
Toss some white sprinkles on the bottom brown portion of the cake and you are done. Enjoy.