Pour the popcorn kernels into your small brown paper bag.
Drizzle oil or melted butter as evenly as possible over the popcorn kernels. Shake the bag thoroughly to coat all of the kernels.
Fold the brown bag shut, and press out most of the air. Secure the bag loosely shut by putting one piece of tape on each corner of the fold and one across the middle of the fold. This should leave part of the fold unsecured but still folded, allowing the steam to escape from the bag.
Put the bag in the microwave and cook on high for 4 minutes. Listen closely to the frequency of the pops. When the popping slows to one pop every two to three seconds, remove the bag from the microwave. This will probably take between two and three minutes, but may be longer.
Open the bag carefully as it may be full of steam.