are you wanting to remove vocals? If so, forget it if you want anything resembling good quality.
If you REALLY want to try it, heres how:
The vocals have to be panned direct center for this to work. You take the left channel and the right channel separately at equal levels and invert the phase of the left channel. Then, you reverse the L/R to R/L. This removes anywhere from 0-50% of the vocals and leaves massive amounts of the music missing and sounding terrible. What this does is cancels out the vocals (the ones panned dead center). THAT means that the two soundwaves are complete opposite of each other during a cycle and equal zero, thusly, no sound. Since you are working with left and right separately and only one is inverted, only what is panned center is removed (not only vocals, but whatever the engineer panned dead center.
If you dont understand what I just said, dont attempt.