Need help merging a song with the karaoke version?

I'm currently testing the voicebank I'm working on. The song's been done already and all I need to do is to add the karaoke version for the background music.

Can anyone tell me how to do it? Help would be much appreciated and thank you in advance.

This tutorial made by OnlySoaa explains very well how to do it with the program called Reaper.
You have a link for the program in the youtube description (it's shareware but once the time has expired you can still use it normally).


If you feel like using Audacity, let me point you to this other tutorial.
I try in the first step to explain it with as much detail as possible... maybe too much. Feel free to ask for more help if you think it's too complex.

Thank you ireitas! I'd be taking a look on it.