Open Audacity and click "File" in the top right-hand corner of the screen. Scroll down to "Open" on the menu that appears, and search for the desired track. Highlight the desired song and click "Open." A small dialogue box will appear in the center of the screen, with a loading bar that fills up as Audacity analyzes the file. The waveform for the track will be displayed when it has loaded.
Click on the small arrow beside the track name to the top left of the waveform display. Select "Split Stereo Track" near the bottom of the menu that pops up. This breaks up the stereo track into two separate tracks, one for the left speaker and one for the right.
Double-click the bottom track to highlight it. The bottom track should now be a slightly darker shade of gray than the top track. Click "Effect," which is located near the top of the screen beside "Analyze." From the "Effect" menu, choose "Invert." (This menu is in alphabetical order.) Wait for a moment as Audacity applies the effect to the selected track.
Click on the small drop-down arrow beside the bottom track's name. The same menu you used to split the stereo track will appear. Notice that a few previously grayed-out options are now accessible. The "Right Channel" menu item will have a tick next to it. Click on "Mono" to change this to a mono track.
Click on the drop-down arrow beside the top track's name. This will have the same options available; click "Mono" for this track as well. The inverted track and the non-inverted track will be played through the same channel, usually resulting in the vocals being removed. Generally, all other instruments will be unaffected.
Click the "Play" icon at the top of the window to verify that the vocals have been removed.