Video Star is a free app that can work on an iPhone or iPad. It is the simplest way that I have found to make music videos. You can use the music they have, or you can insert your own from your iTunes. The app is BEYOND easy to use. When you hit record, it will countdown for you and start playing the music. You can talk in the background and it doesn't pick up any of the noise. This is perfect for teachers because we can instruct our students while you are recording! You can start and stop recording as often as you would like and pick from several background choices. When I have made videos with my class it takes us 30 minutes tops.
I have used Video Star to make several different types of videos. I often use it to make ones related to math. My students learned Doubles Doubles by Harry Kindergarten, so I thought making a video from that would be fun. Enjoy it below!