To air-pop popcorn, follow the instructions provided by the manufacturer of your popping machine (omit oil).
Stovetop
Heat 1 tablespoon oil in a medium pot over medium-high heat. Add 1/4 cup popcorn kernels and cover with a tight lid. Shake the pan frequently until most of the kernels have popped, or there are 3 seconds between pops.
Microwave
Place 1/4 cup popcorn kernels in a small paper bag. Pop on 100% power for 1 1/2 to 2 1/2 minutes, until most of the kernels have popped, or there are 3 seconds between pops (omit oil).