Step 1. Grab your empty 2 liter bottle.
Step 2. Remove cap.
Step 3. Place to lips, keeping ridge of opening level to the ground.
Step 4. Blow.
Optionally, have a friend repeat, but partly filled with fluid, or have one of your own.
You get better sound if you hold onto the hard plastic vs. the soft.
I have an idea for a little amplified electronic touch ribbon violin-type instrument that wouldn't be too hard to build if you have some soldering skills. Could run off of one or two 12v batteries.
I'll post details when I get around to building it, which I think this thread might've inspired me to finally do.