Import a stereo music file into Audacity by clicking "Project" in the top menu bar and then selecting the "Import Audio..." option. You should see two blue lines representing the right and left audio tracks.
Open the "Track" menu by clicking the black down-facing arrow next to the track title in the menu left of the blue tracks, and choose "Split Stereo Track." You will now have a small menu next to each track and can modify them individually.
Select the lower track by clicking in its left menu. The menu and track will turn a darker gray.
Choose "Effects"' from the top menu bar and then "Invert."
Set both tracks to "Mono" by click the down-facing arrow next to the track titles in the menus to the left of each track.
Test that the tracks play without vocals, and then click "File" in the top menu bar and "Export selection as MP3..."
Visit the Audacity Wiki website for further advice. The above technique only works with certain stereo tracks. The wiki can help with others.
Save the song's lyrics as a .txt file. You can find many lyrics at sites such as Lyrics.com (www.lyrics.com/) and A-Z Lyrics Universe (http://www.azlyrics.com/).
Run a CD+G authoring program such as Karaoke CD+G Creator or Karaoke Home Producer to create the graphic lyrics and combine them with the music file. Neither program is free, but they do offer free trials. This way you can choose the program that works best for you.
Burn your completed CD+G files to a disc to bring them with you to the next karaoke night.