Press both flattened hands together in front of your face. Hold the end of your nose with the tips of your index fingers, opening your palms slightly in front of your open mouth. Using one thumb click the tip of your nail forwards against the end of a front upper tooth. This should be concealed by your hands. At the same time push the the tip of your nose to one side.
Voila, onlookers wince in horror, especially effective when performed by young children.