I used this site to get my idea then modified it. I made my mermaid cake with 2 from scratch cake recipes. The Heavenly white cake recipe from allrecipes.com and a chocolate cake recipe from Fanny Farmer cookbook. The main part is constructed from a 91/2 x11 cake pan cut into pieces plus one smaller pan for a few left over pieces. The head is a 6 inch round and the hair is cupcakes. I cut arms out of corners of the smaller cake pan.
Frosting is 1 cup shortening to 4 cups powder sugar to 4-5 tbsp water and food color. All of my mermaid cake is frosting. I made the scales on her tale from just putting the frosting in a ziplock bag and snipping the end. I also used a gel writer for the black. The background is constructed from cutting up green and blue fruit roll ups. It took 4 1/2 boxes and about 4 hours to design and construct. So be prepared to put the time in if you want that background. It was a big hit bright and colorful!