Okay, the instructions are in one of Collette Peter's books, The Art of Cake Decorating. I would copy them, but I don't want to infringe on the copyrights. In a nutshell here it is. Bake a 9X13 cake. (red cakes are kind of fun)
You cut the cake corner to opposite corner in a sort of fish shape. (or approximate the body of the fish you are making) Trim the tail end a little so it is a little lower, like a tapper to the fin. Set on your cake board. Cover with your favorite buttercream. Use rolled Fondant to cover your fish cake. I trim so that the tail fin is part of the body. Cut your other fins to match the type of fish you are doing. I use the dull side of a steak knife to make the 'bones' in the fins. for the top fin, usually there are a few spines that are seperated by a dip. I use a round tip that kind of mirrors that. Then I use a large round tip to make the scales, slightly indenting the fondant. I use the same knife to make the gills. After I have that, I attach the fins with water. Roll a rope to shape the mouth. Attach with water. Then, I paint the fish with vodka mixed with the colors. Sometimes, like with black, I use the straight gel color. I have been meaning to get some luster dust to make the bellies silvery so there is an idea for embellishment. On your serving tray (or cake board) use a leaf tip to make it look like it is laying on a bed of lettice. I used red fondant balls to look like cherry tomatoes. I don't show it on this fish, but I have begun to make little lemon wedges out of fondant as well to place on the board. You could also use those candied lemon wedges, but I can't find them since Fanny Farmer went out of business.
I have 3 other fish cakes to post and each one is a little different. For the one at our family reunion I placed the fish on a 12X18 cake. (with proper supports) that was one HEAVY cake!
PM me and let me know if you have additional questions