This is a 2 part HD video tutorial set on how to make your own acapellas from full songs. This technique uses Ableton's EQ8 device in mid/side mode to isolate the vocal and the Multiband Dynamics device in downwards expansion mode to reduce unwanted elements from the instrumental backing.
If you've never tried mid/side equalization before, then part 1 will be a good tutorial to watch as it has application far beyond this technique. It's commonly used in mixing and mastering. Enjoy the tutorial.