Cut headphone wires at the base of each headphone (L & R). You'll only need one of the two lines so pick the one you want to use and cut the other where they converge mid-way down the length of the chord.
Next, strip the wires. You may notice some fiber-like material interlaced with the leads themselves. Strip this stuff out with your utility knife or you will not be able to get a solid connection from your headphones to the speaker (thanks to Richard at my local Radio Shack on that one). Alternatively, you can use a soldering iron to burn this stuff away. I found this method much easier and a bit more effective.