Download the Audacity software for Mac OS X (see link in the "References" section below). Open the program once it's finished installing.
Click "File>Open" and find the song you want to convert into a karaoke song.
Choose "Edit>Select>All" from the toolbar menu at the top of the window.
Click the small, black downward-facing triangle in the track menu. Choose "Split Stereo Track" from the drop-down menu that appears.
Select the entire bottom track by clicking an empty space in the track menu. The bottom track should be highlighted, while the top track should not.
Click "Effect>Invert" from the toolbar menu.
Click the small, black downward-facing triangle from each of the track menus (one at a time), and choose "Mono" from the drop-down menu.
Click "File>Export as WAV." Save the file on your desktop. You can now burn this file to a CD and put it in your karaoke machine or CD player.