Connect an external microphone to your PC. For professional vocal recording, use a dynamic vocal microphone or a condenser microphone. If using a condenser mic, you will need to first connect it to an audio interface (sold in music supply stores) with a Phantom power switch, and connect the interface to your USB port. For other microphones, you can connect using an XLR-to-USB cable or a TRS-to-USB cable, depending on the microphone type.
Open Audacity. Click "File" on the menu bar, select "Import" and then select "Audio." When the navigation window pops up, choose the song file that you want to import and click "OK."
Click "Tracks" on your menu bar, select "Create New" and then select "Audio." A new horizontal bar will appear on your project, directly beneath your imported file. This is your vocal recording space.
Click "Edit" on your menu bar, select "Preferences" and click the "Devices" tab. On the "Recording Device" menu, select your connected microphone or audio interface from the list of options. If recording with a built-in microphone, leave the default settings. Click "OK."
Connect a pair of headphones to your computer. This will allow you to hear your recorded audio file while singing along, but prevents the music from bleeding into your vocal recording.
Click the red "Record" button and record your vocals as your imported file plays through the headphones. When you finish recording, press the square "Stop" button.