Popcorn requires agitation in order to pop: you need to shake it vigorously during the heating process and keep it in oil to raise the temperature quickly. Because of that, you cannot normally pop popcorn in a conventional oven. (Microwaves work better, because they use energy that agitates the popcorn sufficiently; and even then, there are often a number of unpopped kernels.) Luckily, most household ovens come with a range top, which is much more suitable for popping popcorn right.
Set a cooking pot on top of the stove and pour a measure of cooking oil sufficient to coat the bottom (usually about 2 to 2-1/2 tbsp).
Pour a layer of popcorn on top of the oil. There should be enough popcorn to completely cover the bottom of the pot, but not enough that the kernels are stacked on top of each other. Otherwise, the kernels that aren't touching the pot will not be heated properly (they won't pop).
Set the stovetop to medium-high heat and begin to gently shake the pot back and forth, using potholders to keep from burning your fingers.
When the first kernel pops, cover the pot with a lid and continue shaking.
Listen until the sound of popping slows to 2 to 5 seconds between pops, then remove the pot from the heat.
Allow the popcorn to cool for about a minute or so, then add salt, melted butter or similar seasonings to taste.
Popcorn is best consumed fresh. The sooner you eat it after it comes out of the pot, the better. The same holds true with recipes that involve popcorn. If you can, you'll want to transfer it into the other ingredients as quickly as possible after it is popped.