Rosettes are starting to pop up everywhere; I've seen quite a few cakes completely covered with them (when an ombre color scheme is used, they're especially fetching). To pipe rosettes on a cake, you'll want to use medium consistency buttercream. Of course, I didn't have any of that lying around, so I'll show you with royal icing. Use a star tip in the size most appropriate to your project. I used a tip #21, because it's the first one that fell out of the drawer. Hold the tip just above the surface of your project (whether it's a cake, cookies, petits fours, whatever), and squeeze out a star.
shell borders; the key, of course, is to make them uniform and edge-to-edge with each other. Practice rosettes on Nilla Wafers, so everyone can eat your mistakes!