Take the acorn cap so that the open side is up and curl your fingers around it to support it from the bottom. Take your thumbs and make them flat over the top of the acorn cap, pressing the knuckles together. Cover the bottom part of the acorn cap with the lower part of your thumbs, pressing them together no air can escape. Your fingers should leave a little triangular opening at the top of the acorn cap. Press your open lips over your thumb knuckles and blow hard. It does takes a little practice, so be patient. Beth at Acorn Pies has a good picture of how your fingers should look over the acorn cap, if you can't picture what I mean.