Cut off the tip of the bag and place the frosting tip and the base of the coupler inside. Screw on the top of the coupler on the outside of the bag.
Fill the bag with frosting about half way and place a rubberband on top to hold the bag in place.
Hold the tip above the middle of the cupcake and squeeze out the icing to form a star.
Raise tip slightly and form a tight circle around the star.
After completing the first circle, move the tip back toward the center and go around to make a second circle around the inside edge of the first circle.
Slowly pull the tip away at the center of the cupcake.
Notes
Make sure the cupcakes have cooled completely! You don't want your frosting to melt when piping it on top.
If you don't have a piping bag, you can use a large freezer bag. Simply cut the corner off one of the ends and then add your tip/coupler.
I also like to use a 2A frosting tip. It's easy to pipe with and holds a smooth shape with no ridges (see picture below).
Before piping, make sure your frosting is thick enough to hold a point.
If piping with cream cheese frosting, add 1-2 extra cups of powdered sugar so that it will hold it's shape. Also, make sure not to soften the butter or cream cheese too much.
If adding fruit to your buttercream, use freeze dried fruit instead of fresh (it stiffens up better). Simply pulse the freeze dried fruit in a blender before adding to your frosting.