Step 2: Attach flapper to bike
Ok, now you need your bike. Place the flapper on the rear forks (you can put it on the front forks, but the engine on a bike is more at the back), with one end wrapped around it a bit, and the other pointing right in to the wheel. Make sure you put it on the side where the gears aren't. Before taping it down, you should spin the wheel to make sure it hits the spokes. This is what will make the sound.
There's a bunch of pictures for this step, look at them for detailed instructions on taping down the piece of plastic.
I made a mistake when doing this. Make sure the piece of plastic is pointing much more downwards, not up like it is in the pictures. Otherwise it will make a really gay high pitched sound because it is snapping back too much and hitting the spokes.