I've tried a few brands of popping corn and I prefer "Pop Secret Jumbo Popping Corn". It probably does not matter too much, the important thing is to make sure to keep it sealed once it is opened because if the kernels dry out they won't pop correctly. There needs to be a certain amount of moisture content in the kernels for propper popping.

I recommend using a stove top popper instead of using a pot because the whirly-pop mechanism keeps the corn from burining, and the top vents let out the steam that can make the popped corn chewey. There are a few brands, I have a cheap West Bend that does the trick.

I use coconut oil, and I think most theaters do also, it seems to taste the best and it burns clean.
Put 3 tablespoons of coconut oil in the popper on low to medium heat. For my stove this is about 4-5. Let the globs of oil melt and fill the bottom of the popper.
Once the oil is melted, turn the heat up to med-high, around 7 on my stove, and add the 1/2 cup of popping corn and slowly turn the crank on the popper.
In 3-4 minutes the popping should start and after a total of 4-6 minutes it should be done.

Stand up the paper bag and immediately dump the popped corn into the bag.
Leave the bag open and shake the bag up and down so that the corn does not retain any moisture.

It is important to use clarified butter when making popcorn because all the water has been removed. If you use regular butter you will find that the corn immediately becomes soggy.
I found this butter in the seafood section of Whole Foods, but you can clarify your own butter, just do a search, there are plenty of how-tos on the net.
So while the corn is sitting in the paper bag, heat 3 tablespoons of the butter in the microwave for about 2 minutes, or until it is completely melted.
Then drizzle 1/2 the butter on the popcorn, then shake the bag again up and down, pour in the rest of the butter, then shake again.

You don't need to use Flavacol, but it really does complete the experience. You do at least need salt powder and you can make it yourself by grinding up some salt in a food grinder until it is as fine as flour, but Flavacol tastes better. I got my pint at the Virginia & Spanish Peanut Co. in Providence RI. It should last me 10 years or so.
You can add 1/2 to 1 teaspoon of Flavacol. I usually use 3/4 to 1 teaspoon.
Add 1/2 your amount to the bag, shake the popcorn, then add the rest, then shake again.

Find a bowl worthy of your corn, I prefer stainless, but to each their own.

Dump the paper bag of corn into the bowl and enjoy....