Download a vocal-remover program and install it. There are many of these available online, such as Power Karaoke: CD & G Creator, YoGen Vocal Remover and Magic Vocal Remover. These all work on the same principle -- the vocals appear in the middle of the mix. Unfortunately, on some tracks the vocals will be split off into the right and left channels of the mix, and will not be removable. Most tracks suitable for karaoke, however, do have the vocals in the middle of the mix.
Load your desired track into the vocal-remover program. For best results, use files in WAV format. With Power Karaoke, you will need to open the "Vocal Remover" tool. Double-click the tool's icon on the toolbar, which is a microphone beside a "No Entry" sign. In the "Vocal Remover" window, click "Browse" beside the "Input" field and select the desired track. (This process is virtually the same with Magic Vocal Remover, except you don't need to open any supplementary tool; the program itself is the tool.) Set the "Output" to the desired location.
Drag the desired track file into the main window if you're using YoGen Vocal Remover. This is the simplest program to use, in that after dragging the file into the window, you simply click the "Save" icon in the bottom corner to remove the vocals. The file will be saved in WAV format, without vocals.
Click "Remove Vocals" if you are using Magic Vocal Remover. This button is located in the bottom left-hand corner of the program's main display. If you are using Power Karaoke, there are several options that can be adjusted before you start the removal by clicking on the "Process" button in the bottom left-hand corner. Generally, the default settings will suffice, but they may require altering. "Level" controls the amount by which the effect is applied; "Attenuation" changes output volume; and "Bass Boost" and "Treble Boost" help to keep high and low tones prevalent in the vocal-free mix.
Adjust the "Balance" slider if the program doesn't work in its default setting. This allows you to change where the program looks in the mix for the vocals. Stereotypically, this will be in the center, but you have the option of moving the slider to the left or the right to remove vocals from different parts of tracks.