Choose a picture and print or trace a reverse image of the design. Tape to plexiglass or cake board; tape parchment paper over image.
Outline image with round decorating tip (3, 4 or 5 most common) and colored buttercream icing.
Using medium consistency icing, fill in details on design (ornaments on this tree, sunglasses and mouth.
Fill in background color with zigzag; smooth gently with angled spatula.
Outline the design, at least 1 in. all around, with icing in same color as cake.
Place in freezer until hardened.
Remove from freezer. Place on iced cake.