Get an ipad music recording interface that will allow you to insert a cable with a 1/8" stereo jack. These range in price from $60-$200. The interface will probably have only a 1/4" jack. That's fine, just get a 1/4" to 1/8" adaptor. Open garageband on ipad. On the instrument panel, choose to do an audio recording. Connect ipod to ipad via cable mentioned above, through the interface. (Obviously, you must have the karaoke song you want on your ipod). Play the karaoke song from ipod to ipad garageband while recording on the ipad. Trim and effect to your liking.