Fit decorating bag with star decorating tip 18 and fill 1/2 full or less with medium consistency icing.
Hold the decorating bag at a 45? angle at 6:00, with the tip slightly above the surface.
Squeeze hard, letting icing fan out generously as it forces the tip up. To form curve, lift tip up and over the shell as you move tip from 9:00 to 12:00 to 6:00.
Relax pressure as you lower the tip while pulling the bag towards you at 6:00 to form a tail.
To continue the reverse shell border, pipe a chain of swirling reverse shells, with the fan end of each new shell covering the tail of the previous shell. Repeat with another shell about 1/4 in. away from where you ended the previous one in the opposite direction, curving from 3:00 to 12:00 to 6:00.
The look is even fancier finished with a dot or a star at the center of each shell curve.
An alternate way to use this technique is to have one shell on top of the cake and the second shell on the side with the tails meeting at the edge.