You don't need the overpriced, often chemically-laden bags of microwave popcorn; only a brown paper bag (lunch-sized), plain popcorn kernels, and toppings of your choice.
Stand it upright, then microwave on high power until there are about 2 seconds between pops. On my microwave, that's right at 2 minutes. My neighbors' takes 2 1/2 minutes. Plan on between 2-3 minutes.
Do not walk away during this time! There are reports of bags catching fire- so don't let it cook too long!
Top off your popcorn with a drizzle of melted butter and some salt, close the bag, and shake to coat. I prefer 1-2 teaspoons of butter for a bagful, but use whatever you like. It's easier to shake if you used the 1/4 cup of kernels.
You could also use butter spray, or olive oil, or coconut oil, Parmesan, cinnamon/sugar, Seasoned Salt, Ranch dressing mix powder, or cheese powder and a bit of cayenene and mustard powder. The sky's the limit.