Download and install one of the free, multitrack recording applications available online. ProTools Free is a high-quality solution, but may not run on lower-powered PCs. Audacity is a popular choice for the experienced PC users, as its open-source licence allows modification of the software. Kristal is another popular open-source title that supports different audio drivers, making it compatible with a broad range of third-party effects and instruments.
Shut down the computer. Connect the audio interface device using the USB or FireWire cable. Power on the PC and launch the recording application. Open the Preferences menu within the application and select the Audio Interface make and model from the "Audio Device" drop-down menu.
Click the "Insert" menu and select "Create Audio Track." If prompted by a dialog box, select a stereo track. Click the "File" menu and select the "Import Audio from File" function. A dialog box will appear. Navigate to the instrumental file over which you plan to record. Select it and click the "Open" button. Allow the audio file to import and a waveform diagram to be generated.
Play the track to ensure that the instrumental has imported correctly. Repeat the process to create a stereo audio track, creating a second one alongside the instrumental. Connect the microphone to the audio interface and click the "Record Enable" button on the blank audio track.
Test the levels by pressing "Record" on the multitrack software and singing or speaking into the microphone just as you plan to on the final recording. Stop after the first 10 seconds or so, and play back what you have recorded. Increase the microphone gain control slightly if the volume is too low, or decrease it if there is any distortion or "clipping." Repeat the test and listen again. Ideally, the level should be close to perfect before you record the vocal, as attempting to correct any major discrepancies after recording rarely produces good results.