Peel the plantains and cut them into 1-inch thick slices.
Heat the vegetable oil in a large skillet or frying pan over medium-high heat.
Fry the plantain slices in batches until golden brown, about 2-3 minutes per side. Remove from the oil and drain on paper towels.
While still warm, use a plantain press or the bottom of a glass to gently flatten each slice to about 1/4-inch thick.
Return the flattened plantains to the hot oil and fry until crispy and golden brown, about 1-2 minutes per side. Remove from the oil and drain on paper towels.
Season the tostones with salt, garlic powder, and black pepper while still hot.
Enjoy your meal!