Double-click on the "Audacity" icon on your desktop. It may take a few seconds for the program to open.
Click "File" and select "Open." Choose the session in question from the drop-down menu.
Mute all irrelevant tracks, such as drums, guitars and piano. Click "M" on the channel strip next to each track.
Click "Tools" and select "Equalization." The equalizer tool enables you to enhance the bass frequencies and cut the vocal frequencies. While this won't completely remove the vocals, it will diminish them to the point where they'll be less audible in the mix.
Increase the bass slider dials. These are located to the left. Slide them up as far as they go.
Reduce the treble and mid slider dials. These are located to the right. Slide them down to the minimum setting.
Click "Tools" once more and select "Compression." Press play so you can hear the compressor adjustments in real time. Increase the "Threshold" and "Ratio" dials so that they squash the sound. Compressors work by cutting high volumes and enhancing low volumes. The bass and vocal will have recorded at different volumes. Tweak the compressor parameters until you find the "sweet spot" that cuts the vocals but boosts the bass.
Click on the vocal track to highlight it. This enlarges the sound wave graphic, so you can more accurately determine which sound wave peak belongs to which sound as you hear the audio.
Click "Tools" and select the trim tool, illustrated by a "scissors" icon.
Click the cursor either side of the sound wave associated with the unwanted vocal sound. Once you trim the vocal audio, it becomes a separate file. Click on it to highlight it then hit "Delete."
Repeat this process for every portion of vocal to be deleted. To remove an entire vocal track, either delete the whole thing or press the "M" icon on the channel strip to mute it. The latter approach gives you the option to use or edit the vocal audio at a later date.