To make a fish costume for a fancy dress, start by gathering materials like a blue or silver dress, blue or green leggings, and tulle. Cut the tulle into strips and attach it to the dress to mimic the look of scales. Attach felt or fabric fins to the dress and leggings for added details. Use craft foam or felt to create fish eyes and attach them to a headband or a hat. To complete the outfit, use face paint to create fish-like patterns on your face and wear matching accessories like fishnet stockings or fish-shaped jewelry. Don't forget to style your hair to match the fish theme, such as braiding it in a fishtail style or wearing a seashell hair accessory. With these simple steps, you'll be ready to rock your fish costume at any fancy dress event!