The real secret is flavacol.

Amazon sells it in bulk, or you can get this stuff: https://www.amazon.com/dp/B00M8HPGMU/

comes in different sizes. we use the 4oz ones with our stir crazy popcorn maker.

You can buy in bulk and get cheaper with just the popping corn, carton of flavacol, and the coconut popping corn oil, but its a little more work to measure it all out. YMMV.