Looks like a "rose" tip was used, a large one. Hold the bag with the wide end of the tip closest to the cake and the narrow end at a 45 degree angle. Start squeezing and move your hand in a narrow upside down "u". Stop squeezing, and pull away. You will want to apply slightly more pressure in the "curve" for a wider petal. Practice on a board until you get the pressure and motion in sync. HTH
really? I thought it was done with fondant.
I did something similar and used the icer tip, just make sure the jagged side stays down. Just a quick up down motion to make the petal.