Stir popcorn maker something like this https://www.amazon.com/gp/product/B00004RC6R
flavacol salt (optional if you use flour salt) https://www.amazon.com/gp/product/B004W8LT10 or flour salt https://www.amazon.com/Morton-Popcorn-Salt-Green-French/dp/B... or both 1/2-1 tea spoons at most.
'butter' oil https://www.amazon.com/gp/product/B000UZLQG2 1/4 cup after cooking (warm it in a microwave)
The butter oil is a bit of a pain as there is so much of it and you can not really buy in small quantities, and it will go rancid before you use it all. It is meant for movie theaters to use. I have not found any supermarket versions that are correct. Usually they are kinda gross.
You can use a coconut oil for cooking with. Most theaters do not do this at all. The butter oil is closest to the same as movie theater as that is what they use during cooking and topping at the end. I personally use the coconut oil when cooking as it imparts a nice texture and flavor to the popcorn. 1-2 tsps for cooking with for either. Then top with the butter oil.
The flavacol and butter oil are the key ingredients.
For the popcorn I just buy orville redenbacher popcorn. They have a decent process of removing the duds before they get into the container. You can buy generic just expect a few duds.
My favorite is to use beef tallow.