So basically I took an old set of crappy, broken headphones and turned them into really cool looking (and functional) tennis ball headphones (I was inspired by a tennis ball stereo instructable).

Here's what you need:
a tennis ball
a saw
an old set of crappy, broken headphones (speakers and wire still intact)
a hot glue gun
a metal clothes hanger
string
some sort of cloth (the softer the better, but not too thick)

Step 1: Cutting the Tennis Ball and Sizing the Speakers

Ok, so the first step is to cut the tennis ball in half. I found that the easiest way to do this was to simply stick the tennis ball in a clamp, to hold it still, and then to saw it in half. Also, try to make the tennis ball halves the same size.
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

Now that your speakers will fit into the tennis balls, you will need to position them so that they feel relatively comfortable and get a good sound. It may take a while for you to position them just right.

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

The next step is to cover the speaker with cloth so that it feels comfortable when you put it up to your ear. You might want to have a pretty soft cloth, but make sure it's not too thick (you don't want the sound to get muffled).
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)

This step is probably the hardest so far. First, you have to take your clothes hanger and bend it into a straight line. Then bend it so that if fits over you head. Finally take the ends and make a cup-like form (see picture). Be sure to clip off the excess wire.

Step 5: Last Step

This is the final step. In this step you have to attach the speakers to the hanger wire. I'm not quite sure the best way to do this step, but I'll show you what I did.
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.