It's easy, either have a 1/8' male to male cable outputing from the headphones jack into the microphone jack, that'll work.
Or, you could just go to control panel--->sounds and audio devices---->click on advanced in the device volume square, this will open up the volume control window---->options---->properties, select the recording circle, make sure line-in is check marked (there might be variations to the exact name). Press Ok. Now you'll be back at the volume control screen, and you'll be given the recording options, select line in.
Now your set to record what ever your soundcard outputs. Go to a recording program, cool edit pro maybe, and press record, and then press play on whatever you want to record.
Hopefully this helped.