Pick a track and drop it into a new Audacity project.

Next, find some sick voice-over to drape on top. There is no better place to start than than the Archive.org Audio section. When you find a song you like, download the mp3 version. You might have to right-click on the link and select Save Link As… or Save Target As…

Now you need to click Project and choose Import Audio to make a new track with the voice-over on it.

Depending on the length of your track, you might need to import multiple files or loop one file two or three times.

Download an example of voice-over