Pastry bags are available in many sizes and types, and each has strengths and drawbacks. Canvas pastry bags, for example, handle stiff applications well, but they're difficult to clean. Nylon pastry bags are easy to clean, but they often become slippery and don't hold up well. Plastic-lined cloth pastry bags are handy for various piping jobs, but you can also use a Ziploc bag or another brand of plastic baggie to apply frosting to cupcakes. In addition, since it's a low-cost, disposable bag, you save time and money.
Fold the top of a Ziploc bag -- the bigger, the better -- about halfway down over the rest of the bag to help keep it open. Spoon the frosting into the bag so it's approximately half full.
Unfold the top of the bag and twist it to close it tightly. Grip the bag right beneath where it's twisting closed with your free hand. This squeezes air out of the bag and compresses the frosting down into one corner of the bag.
Hold the bag closed beneath the twist and use your other hand to snip off the corner of the bag where the frosting has accumulated. Cut only about 1/4 inch from the tip of the corner, as too big of a hole will release too much icing and you will not be able to apply it neatly.
Position the hole about 1/4 inch above the center of a cupcake, holding the bag with your free hand perpendicular to the surface. Use the hand holding the bag closed to gently squeeze the bag, forcing the frosting through the hole. Deposit a dollop of icing onto the center of the cupcake.
Slowly slide your hand down the bag as you continue squeezing the frosting out while slowly guiding the tip of the Ziploc bag in a circular motion from the center to the edge of the cupcake. Continue around the edge, then slowly circle back in toward the center once you get all the way around. Lift the bag gradually higher as you move inward to work up to a peak.
Stop squeezing the frosting out through the hole when you get to the top of the frosting at the center of the cupcake. Pull it straight up, then move it away.