Bake a sheet cake the size that you need. Using an icing filled bag and round tip, size #5, draw the outline of Scooby Doo on the cake.
The nose of Scooby is made from pieces of the cake that were cut away from the outline.
Looking at the picture, you can see how Scooby Doo is cut from the
rectangle of the sheet cake with the nose pieces added on from the cut
away pieces.
Cover all of the cake using star tip, size #18.
First fill in the eyes with black and white icing. Fill in the inner ears and the tongue with pink icing. Fill in the nose and mouth background with dark brown icing. Fill in the body with brown icing.
Make the outline using dark brown icing and round tip, size #5.
The whiskers are cut pieces of black licorice.
If you are unsure about drawing the Scooby Doo outline on your cake (which is the hardest part), you could use this scooby doo cake pan to bake your cake in. It is not the same shape as the one shown above, but it might be easier to already have the Scooby shape to decorate.