Fit the decorating bag with tip 21 and fill 1/2 full with medium consistency icing. Hold the decorating bag 45° at 6:00; the tip should be slightly above surface.
Squeeze bag hard letting the icing fan out generously as it forces the tip up.
Gradually relax your pressure as you lower the tip until it touches the surface.
Relax the pressure and pull tip away without lifting it off the surface-to pull the shell to a point.
To make a shell border, start the second shell about 1/4 in. away from the tail of the previous shell so that the fanned end just covers the tail of the preceding shell to form and even chain.