My nephew wanted a Ninja Turtle Birthday Cake, so I looked up pictures on the internet and made a template out of cardboard.
I used a rectangular cake tin and made a basic plain chocolate cake. (You can use 2 packets of Betty Crocker cakes if you want). After it cooled, I cut the shape as shown by placing the cardboard template on the cake. Then I used green food colouring to colour Betty Crocker frosting into two shades of green.
To make the headband, mouth and eyes, I used sugar paste. Icing made from icing sugar and water was tinted black with edible black pigment and used for eyes and outline.