Cut the chiles into small strips with scissors. In a clean coffee grinder, grind the strips in several batches until powdered. Grind the cumin and chile seeds in the coffee grinder. Combine the powdered chile, ground seeds, Mexican oregano, and garlic powder in a mixing bowl. Grind the coarse powder in batches in the coffee grinder until fine, about 2 minutes. Store in an airtight container until ready to use.