If you want to start at the beginning of the series, there's a playlist you can follow:    • Understanding Frequency Modulation  

There's a simple little technique just waiting to played with on almost every oscillator feeding the output back into the FM input. We'll see what happens in this video.

There is also a second channel video that extends this one, looking at Triangle & Pulse waves, not just the sine wave here.    • A bit more selfmodulation