Here's what you need:
a tennis ball
an old set of crappy, broken headphones (speakers and wire still intact)
a hot glue gun
a metal clothes hanger
some sort of cloth (the softer the better, but not too thick)
Step 1: Cutting the Tennis Ball and Sizing the Speakers
Once you half your two tennis ball halves, you'll probably want to make a notch near the bottom of each (for the speaker wires). I used my pocket knife to do this step.
Now you have to make sure your headphone speakers will fit into the tennis ball halves. You can do this one of two ways. I simply left my speakers in their case and sawed off the part that connects them (see picture), but you could also just take the speakers out of the case (if you do this, the speakers will be smaller and will sit farther back in the tennis ball; the speakers will be farther away from your ears).
Step 2: Position and Attatch the Speakers
After that, take out your hot glue gun and glue your speakers into the tennis ball halves. Also glue your wires into those notches you made in the tennis ball.
Step 3: Attatch Cloth
Trace the contour of the tennis ball halves onto the cloth and cut them out. You should now have two pieces of cloth that fit perfectly over the tennis ball halves. Now, simply put hot glue around the edge of the tennis ball halves and apply the cloth.
Step 4: Making the Part That Fits Over Your Head (whatever the Heck That's Called)
Step 5: Last Step
I didn't want the speakers to be permanently attached to the hanger wire, so I used string. I think in the future, however, I would like reattach the speakers using some other method (if you have suggestions I'd love to hear them).
So, I took four small pieces of string and hot glued them onto the tennis ball halves so that they fit over the wire. When finished they could slide just a little bit, thus making the speakers adjustable.
The sound quality of the finished product largely depends on the quality of the original speakers. Mine turned out to be a pretty nice pair of speakers.