The rectangular piece of cardboard is for your uber-technical tool. Cover the rectangle in 2 layers of foil. Voilà. You have your tostonera or Plantain Press, traditionally made of wood.
Cut the ends off and score the plantain down the middle from top to bottom. With a butter knife gently pry the peel off. Cut the plantain into rounds.
Next, fry them in some oil, turning to brown evenly.
Place on some paper towels to drain.
While the second batch cooks, you can start using your tostonera to flatten the rounds into their distinctive shape. Keep these to the side while you continue frying all the rounds.
Just like Belgian fries, tostones are fried twice. After pressing, they go back into the pan just long enough to brown evenly.