Open Audacity. If you want to import an audio file containing your background music, click (File > Import > Audio) on your menu bar. If you want to record your melody line from scratch, click the "Record" button on your Audacity window and begin playing your instrument. The "Record" button appears at the top of the window as a bright red circle.
Click "Tracks" on your menu bar. Hover your mouse over the "Add New" option and click "Audio Track," which appears on the second drop-down menu. A new horizontal work space will appear on your main Audacity window. This is the recording space for your vocals.
Click anywhere inside the new track space to select it for recording. A yellow outline will appear around the track space, indicating that you can begin your recording.
Connect a microphone to your computer. You can use your PC microphone, but you will not get studio quality. If you want to connect a professional XLR microphone, you can purchase an XLR-to-USB cable from your local music supply store.
Click "Audacity" on your menu bar and select "Preferences." Click the "Devices" tab and select your microphone from the "Recording Device" sub menu, near the bottom of your "Preferences" window. If using your computer's internal microphone, select "Built-in Microphone." Click "OK."
Connect a pair of headphones to your PC. This will allow you to monitor the original recording quietly while you sing along, without adding unwanted feedback to your new vocal recording. Put the headphones on before proceeding.
Click the "Record" button and begin recording on your new track as the original recording plays. When you finish, press the "Stop" button, which looks like a yellow square. If you make a mistake, click the recorded region (which looks like waves inside the track space), click "Delete" and try again.