Prepare a piping bag with a #21 Wilton tip (a moderate sized open star). This is the tip I used, but other open star tips could also work. Fill piping bag with the darkest color icing. Remove cake from freezer, and start along the bottom of the cake, creating rows of swirls. For each swirl, start in the center of the "rose" shape and pipe in a spiral around the center, about 1 1/2 rotations. Continue along the bottom of the cake, creating rose-like swirls. Repeat with remaining colors of icing. You may find that there will be some areas in each row that don't get fully covered, to fill in the little empty spaces just squeeze in some icing, in a curved shape (like a "C") to mimic the rest of the swirls a bit. For the top of the cake (shown below) I used a combination of swirls and some "C" curves for a looser, wavy effect.