In advance: Make cookies. Prepare and roll out dough. Cut fish using cutter; turn upside down to match fish as shown. Cut mermaid tail using knife and pattern. Bake and cool cookies. Decorate fish with color flow icing. Outline with full-strength icing and tip 3; flow in with thinned icing. Let dry. Pipe tip 6 dot eyes, matching bead lips and contrasting bead details; add tip 3 dot pupils. Let dry on waxed paper-covered boards.
Bake and cool cake. Use knife to level foot area where fin cookie will rest. Ice sides and background areas smooth with blue icing; pipe tip 18 shell border. Outline face, both arms and torso with tip 3; fill in with tip 16 stars, overpiping nose for dimension.
Attach cookie tail with icing. Cover end of tail with overlapping tip 127 lines; work from outer edge inward with wide end of tip up. Pipe tip 124 petals for scales. Pipe tip 32 swirl hair. Pipe tip 18 elongated shells for bikini top. Pipe tip 3 outline smile and tip 6 dot eyes (flatten slightly with finger dipped in cornstarch). Position fish around cake.
* Combine Lemon Yellow with Golden Yellow for yellow shown.
* Combine Violet with Rose for violet shown.