Trim the baked cake layers flat so that they are ready to start stacking. Cut out two, 4-inch cakes from the loaf. A large cookie or biscuit works well if you have ones. Trim one of the 6-inch cakes so that it is 5-inches in diameter.
Begin stack the cake layers, starting with the 6-inch cakes. Add about ½-inch of buttercream between each layer, or even a little less. You don't wan't the buttercream layers to be too thick or it will be hard to shape into the 3D tree.
After you stack all the cake rounds, chill the cake in the refrigerator for at least 15 minutes. This will help with the carving.
Use a serrated knife to start shaping and carving the cake into a cone. Angle the knife up the side and trim. Save the trimmings.
Use some of the cake trimming mixed with a bit of buttercream to make a cone that forms the top of the tree.
Once you have your tree shape, cover the cake in a thin layer of buttercream.
To decorate, tint the buttercream green. Use a star tip (I used Wilton 4B) and pipe shells around the tree. Start at the bottom and work your way around and up the tree. Add sprinkles as you go. Once the buttercream dries, they won't stick very well.