Load the video file containing your desired footage into the video editing software. This may require using an "Import" option in your menu bar or clicking and dragging the file icon into your video editing software.
Silence or delete the audio from your video file. Mute its audio track or remove it altogether to make sure it does not interfere with the song you will be mixing into the composition.
Load the audio file you wish to mix into the composition. Make sure the audio has already been edited to your liking, as most video editing software does not feature audio editing capabilities comparable to audio editing software such as ProTools.
Synchronize the audio clip or clips to match your audio. Slide the video and audio clips along their tracks accordingly until they match during playback.
Export the session to mix it into one file. Find the "Export" option in your menu bar; it should be close to the one for "Import." You will have successfully mixed a music video once the composition has saved in the video file format of your choosing.