Although you can do this with already roasted peanuts, it's better with raw peanuts. If you get peanuts in the shell, take off the skin, too, when you shell them.
2 tbsp. vegetable oil
2 cups peanuts
1-1 1/2 tsp. chile powder
1/2 tsp. salt
In a large skillet, heat oil over medium heat. Add peanuts and cook, stirring, for 2 minutes. Drain peanuts on paper towels and while still hot, place in a bowl and toss with chile powder and salt.