Heat the oil in a large skillet over medium heat. Add the plantain slices and fry until they soften, 5 to 10 minutes. Remove from the oil and drain on paper towels.
Use a tostonera (a press) to slightly mash each piece until it is about 1/2 inch thick. If you do not have a tostonera, use a mallet or place the pieces between a folded paper bag and press down with a saucer. Press all of the pieces before going onto the next step.
Return the pieces to the hot skillet and fry until crispy, about 3 minutes per side. Drain on paper towels and season with garlic powder and salt while still warm.