Understand how the vocal cords work!
12 ounce sturdy plastic soda bottle
9" balloon (will be the "diaphragm")
12" balloon (will be the "lung")
Pull the first balloon tightly over end of tube so that it covers about half the opening.
Pull the second balloon tightly over end of tube to cover the other half of the opening and just barely overlap with the first balloon.
Wrap rubber band around balloons to secure in place
Trim balloons so there aren't ends hanging off (optional)
Test your vocal cords by blowing hard. If it doesn't work, readjust the balloons (most common problem is that the balloons aren't pulled tight enough).