Listen to the background music, at some point (Usually every 2 bars), the music will switch up and loop. You want to get the timing of that music, then just copy it onto the end of the track and move it around till you get it looping right. Its doesent have to be amazingly exact as you can always just play a light instrument sample to not only add to the beat but also blur over any small mistakes you make.

This is assuming your using cool edit pro like everyone else.