Place the buttercream in a pastry bag with a #3 small round tip. Hold the bag at a 45 degrees angle.
With the tip lightly touching the surface of the cupcake, start to squeeze the bag with a steady, even pressure. Lift the tip off the surface to allow the icing string to drop. Stop the pressure and touch the tip to the surface to attach the icing string. Pull away the bag.
To write, steadily squeeze and glide along the surface in a smooth continuous motion, keeping your wrist straight and moving your entire forearm in a single unit. Use your arm, not your fingers, to form each line, letter or word. Repeat.