Select the karaoke mp3 over which you wish to record vocals from your computer.
Open the selected karaoke mp3 with the audio recording software of your choice.
Insert the 1/8th-inch end of the microphone cord into the "mic" input on your computer's sound card.
Click on the "record" button in the audio recording software to begin playback of the karaoke track and recording.
Sing over the karaoke track and adjust the microphone levels until the desired level is reached.
Click the "stop" button when the desired microphone level is reached.
Click the vocal track to select it and delete the vocal track.
Click the "record" button again to begin playback and record the vocal track with the new microphone level.