The Cornet: A cornet is a small piping bag made from parchment paper. It is usually used to make fine decorations.
Cut an 8 by 12 by 14 1/2-inch triangle from a sheet of parchment paper. Hold the middle of the long side of the triangle between two fingers of one hand. Take the tip of the triangle on the short, wide end and roll it toward the other tip of that same end while simultaneously pulling it in an upward motion. The tip of a cone will form where your thumb and finger hold it on the long side.
Release your grip from the long side, so that you are now holding the two corners where they meet. The paper will already resemble a partially formed cone. Roll the remaining tail until it is completely rolled into a cone. There will be one point sticking up from the open end. Fold it inside toward the center, and crease the fold. Now you should have a cornet. To close the cornet once it has been filled, fold it away from the seam; this will keep the seam from opening. Use a pair of scissors or a sharp paring knife to cut an opening at the tip of the cornet to the desired size.
From Dessert Circus, Extraordinary Desserts You Can Make At Home by Jacques Torres