I want to create a karaoke version from existing songs. What technique can be used to remove the vocals from the songs?
The simple answer is - none.
There have been many bits of software over the years claiming to be able to do it, but frankly, unless the vocal is clean, dead-centre & has no effects on it, you're not going to really get anywhere at all.
The purported method is to split the stereo track, then phase-invert one side; which sounds horrible & only vaguely works anyway.
This page has a list of software purported to be able to do it… http://mp3.about.com/od/essentialsoftware/tp/Top_Vocal_Removers.htm
I'd recommend trying any free ones, but don't pay for one unless it's got a trial period so you can test it first & decide for yourself.