This is my first instructable, so I thought I'd do something simple. Sorry for the lack of any actual photographs; my camera lost its will to live and dove off the table... I hope my crude 'Paint' drawings will suffice.
This may be common knowledge. Feel free to let me know if my instructable is completely worthless. Here we go...
Step 1: Rectangulatin'
You'll need a small rectangle of paper. Size and precision are not important here (you don't even need scissors, just tear it out). 2 inches by 4 inches works well.
Step 2: Halves and Quarters
Fold the paper in half width-wise, then fold the ends to the center crease. You should end up with something like this:
Step 3: Notch It...
Tear a couple of notches in the center crease. Size and number of notches are not crucial...
Step 4: Blow!
Place the center crease between your index and middle fingers so that the center section extends outward. Press against your mouth as shown. Blow! You'll have to experiment a little with the gap between your fingers, but when you get it right, it'll scream! These are VERY loud.