Cut the plantains into 4-5 thick round shapes.
In a large saucepan, pour vegetable oil until it's at least 3 inches (or about 5 cm) deep. Place on medium heat until the oil is 375℉/190℃.
Gently drop the plantain slices into the oil. Do it in batches, so you don't overcrowd the pan.
Fry for a few minutes until lightly brown (3-4 minutes on each side).
Using a large slotted spoon or something similar, remove the plantain from the oil to flatten them.
Place a tostón on a chopping board or flat surface, and press down on it firmly with the back of a heavy plate, bottle, or skillet until flattened to about ¼-inch thick. Repeat with the remaining slices.
Fry the flattened pieces in the 375°F/190℃ oil, turning them occasionally, for 2-3 minutes or until they are golden. Then transfer them with a slotted spoon to paper towels to drain, and season them with garlic salt and pepper.