Requirements:
Audacity - (Or) any MP3 editing tool that allows you to import tracks on top of eachother & with a panning feature
Adobe Audition - Any version should be fine, but version 3 onwards would be a safe start
Kn0ckout.dll
U need two sources (instrumental & song of course) of the exact same file, bitrate & what not for it to work. Two different audio quality's won't work. So it's generally the best idea to have the instrumental & song from a single or something. Or perhaps the song & instrumental from Spotify, just somewhere where both versions would be the same audio data
Open up audacity, allign the instrumental & song perfectly, pan the instrumental full to the right (I forgot - should be right, if not just do it the other way around), song to the left. Save the Wav/MP3, open up Adobe Audition, you need
the knockout.dll for Adobe. Import the MP3/Wav from Audacity, open up Knockout from the effects tab - & the magic happens. Play around with it. Doesn't work all the time, there's tons of methods, that's just the generic one. Depending on the songs instrumentation & how many tracks it contains, it can come out clean or like ass, but all you need is for the vocals to be mixable.
Anyway, it's funny that you ask when there's like, a kizzilion tutorials on youtube by now on how to make diy acapellas
Good luck.