You get the song and the instrumental. Both have to be the same quality (i.e. 320kbps). Throw them in a DAW or sound editor and line them up so they're both perfect aligned. Then you reverse the polarity of the instrumental, and voila! You'll know you haven't lined them up perfectly if after the last step there is no difference, in which case you'd go back and adjust the alignment of the song and instrumental.
Another tip: In the case that you cannot find an instrumental of the same quality, or not at all, you can use the song it self!
Of course this would only work if there's a point in the song where there are no vocals (for example a song that has an intro, outro, or bridge, 4 - 8 bars, all music, no vocals.) An example off the top of my head would be "HUMBLE" by Kendrick Lamar.