Swirly icing, used to decorate cupcakes or for decorative elements on frosted cakes, is made by using a star shaped nozzle in a piping bag. If you don't have a reusable piping bag, you can still use a nozzle with a disposable piping bag. If you don't have a nozzle or a piping bag, you can still cut a regular zippered plastic bag to achieve swirly icing.
Disposable Bag Method
Slip the star nozzle into the end of a disposable piping bag so it sits right at the end.
Mark with a marker a 1/4-inch from where the nozzle ends, then remove the nozzle from the bag.
Cut the tip of the bag off at the 1/4-inch mark, and reinsert the nozzle, which will poke out the tip of the bag so you can pipe on swirly icing.
Zippered Plastic Bag Method
Wrap a piece of sticky tape around one corner of a zippered plastic bag. If you don't have a zippered bag, any bag made from thick plastic (not a regular shopping bag) will work.
Wrap a second and third piece of sticky tape around the corner of the bag so the tip is reinforced.
Cut the tip with the scissors to make a slit around 1/4-inch long in the end of the tip, then make another slit in between the first two, so the tip of the bag has a cross slit into it. The cross slits will make the icing come out in a swirly pattern like a star nozzle would.