Using this site, and the mermaid cake instructions from Family Fun, I made my daughter a Mermaid Birthday Cake Design for her 5th birthday.
She wanted it to have "red hair like Ariel," blue eyes and a green tail. Other than that, I was on my own. So, I decided that she needed arms, hands, a nose and more decoration than the Family Fun cake. I baked three cake mixes - 2 chocolate and one yellow cake. I made one into 24 cupcakes (tail and hair), one a 9 x 13 (torso, fins, arms) cake and one an 8-inch round (head) and 12 additional cupcakes (hair and hands with leftovers). I ended up with 8 unused cupcakes and several chunks of the 9x13 after carving it into the torso, fin, and arm pieces.
I made two batches of Swiss meringue buttercream - one for a crumb coat, skin-colored frosting, and to stick the cupcake-tail together, and one for the red hair and green tail. I had about 1/2 a batch of that left over.
I used 16 cupcakes for the tail, one for each hand, and 10 for the hair. The hands could also be carved out of the 9x13 pieces. I simply piped on frosting and smoothed it out to cover, or piped on in a pattern for the tail and hair. The scales on the tail are 3.5 packages of Necco wafers, overlapped, then sprayed with Wilton green color spray. I sprayed the whole tail, then added green piped-on frosting for fin detail. The kids sprinkled gold edible glitter stars (Wilton) on the mermaid's hair and tail. Her top is purple Skittles, and the necklace is strawberry milkshake Whoppers.
Bracelets are more Skittles, and we added some edible flowers (Nasturtium, Borage, and Squash blossom) and shells for fun. I used pink Laffy Taffy for lips and one Necco and one blue Skittle each for the eyes (frosting for eye lids and chocolate shavings for eyelashes).
The cake was a huge hit, and we sent everyone home with birthday cake since this was easily large enough to have fed at least double our party of 18!