Piping frosting can be tricky and takes experience. I like to practice on one or two "testers" or parchment paper first. Take a piping bag with a tip, fill it up with frosting, close off the end with a pastry/bag clip, hold the bag with two hands (one at the base near the tip and one at the back near the end of the frosting. Gently squeeze from the back hand and guide with the hand closest to the tip. Move in a circular motion going from outside the cupcake to the middle.