What's a backing track and how do I make one?
A backing track is a recording of a song with all the instruments, but no lead vocals. It usually includes drums, piano and/or guitar, synths, and bass. Many different recording programs allow you to create songs using built-in synthesizers and drum machines and to record yourself playing instruments.
An inexpensive and easy-to-learn option is GarageBand by Apple (macOS, iOS).
You'll need to convert any uncompressed .wav or .aiff files into .mp3 or .m4a before uploading.