Download and install the Audacity sound editor (see Resources).
Open Audacity and click "File > Open" and locate the song you want to convert to a karaoke track (start by browsing through your "My Music" folder if you're not sure where your songs are stored).
Click the small, black down arrow located in the track menu, and choose "Split Stereo Track."
Left-click an empty area in the bottom track menu (which has been separated) to highlight the entire track. You should be able to tell it's highlighted because it will be a darker gray color than the top track.
Click "Effect > Invert" from the toolbar menu.
Click the small, black down arrow and select "Mono." Repeat this step with the top track.
Select "File > Export as WAV" from the toolbar menu. Save the item to your desktop and name it the title of the song. This track has now been converted to its karaoke form.