Make two fists and press your knuckles together. Rest the acorn cap on your fists with the open side up. Cover the bottom part of the acorn cap with the lower part of your thumbs. Try to press them together hard enough so that no air can escape that way. You should leave a little triangular-shaped opening at the top of the acorn cap.
Press your open lips over your thumb knuckles and blow hard.
It takes some practice but an acorn cap makes an incredibly loud whistle. Don't blow the whistle near people's ears!
What else can you use to make a whistle? I've tried a pen cap, a bottle cap, and a slipper shell. Would a water bottle cap work? A walnut shell?