For me, I'm pretty sure it's related to having just awful seasonal allergies as a child - I'd quite often get stuffed up and have an unfortunate amount of pressure in my sinuses and ears. So then I'd pinch my nose and try to adjust it, eventually realizing I didn't need to pinch my nose.
I can do it on command but never paid attention to how before.
It's basically "yawning manually". There's a couple different muscles activating at the same time, most irrelevant to the popping, I can't control them individually yet. (Probably learnable, if I practiced I could probably get the pop more efficiently.)
But yeah, just yawn real wide, that should do it.
A lot of stuff is learnable, like I couldn't move my ears but taught myself how to do it because I was bored.