Ohhh Greg, you do realize that not every single word you say must be screamed at the top of your lungs don't you? You are by far the single most annoying thing that's ever come into this fucking house. SHUT. THE. FUCK. UP. Inside voice it up, bro!
First, you take the normal audio track of the song, then you take just the instrumental with no vocals on the track and play them together. From there you invert the instrumental track, which reverses the audio signal upside down and that cancels out the instruments all around, leaving you with just the odd audio left standing out, which is the vocals :D