Heat the popping oil in a large, heavy-bottomed pot with a lid over medium-high heat. When the oil is shimmering, throw in a couple of kernels and cover the pot; when they pop, you're ready to go!
Add the rest of the kernels, cover tightly, and, holding the lid to the pot, shake over the heat continuously until all of the kernels have popped.
Remove from heat and empty popcorn into a large bowl, removing as many unpopped kernels as you can.
Combine the oil with any seasonings you desire. If you're using a powder (like ranch seasoning), plan on about 1/4 cup of powder per quart of popcorn, reserving the last quarter cup. Toss the seasoned oil with the popcorn gently but thoroughly, so that all of the popcorn is coated. Shake on the reserved powder (or any heavy add-ins, like shredded coconut or sesame seeds) and toss to combine.