Fit decorating bag with petal decorating tip. Fill decorating bag 1/2 full or less with icing.
Hold the decorating bag at a 45? angle at 3:00 (9:00 for left-handed decorators), with wide end of tip lightly touching surface and narrow end facing down and away from the surface.
As you keep the wide end of the tip against the cake, move your wrist up to pull up the icing; move wrist down to complete one wave of the ruffle.
Repeat this up and down motion as you move your arm along the surface of the cake for the entire ruffle.
To end the ruffle, bring tip back to the surface, stop pressure and pull tip away. Overlap the first ruffle wave with the last ruffle wave if making a continuous, circular border.