I've literally never gotten those clouds traveling the planet... need to research what creates them more closely and chase them.
Well, first you need a thunderstorm. Reading on Wikipedia, I was surprised how widespread mammatus clouds are. On the other hand, we have plenty of thunderstorms here but I probably see mammatus only once a year or so. And hardly ever during sunset.
Thanks for the comments, everyone. I would have liked to make the farmhouse smaller but there was a line of grain bins behind me.