Trim both ends off the plantains and score the skin down one side, trying not to cut the plantain below as far as possible. Pull away from the ends down the sliced skin to peel off the skin. Cut the plantains into round slices (crosswise) about ¾ - 1in (2 - 2.5cm) wide.
Add a layer of oil to a small skillet/frying pan, around ½ in (1.5cm) deep and warm over a medium to medium-high heat. Prepare some kitchen paper on a plate, ready to drain the plantain on. Once warm, fry the slices of plantain in batches, taking care not to overcrowd the pan. Cook for around 2 - 3 minutes each side until they get lightly golden then transfer to the kitchen paper. Repeat with the rest of the slices.
Once you have cooked all of the plantain slices, turn off the oil for a couple minutes while you get the plantain ready for it's second cook. You want to flatten each slice to around ¼in (½cm) thick - the traditional tool is a tostonera, but you can you a tortilla press or, as I do here, just a small wooden board or eg a mug or jar with a flat base. I recommend covering at least the top, if not also the bottom, with a small sandwich bag (or you could use cling wrap/film but it is more likely. to break) to help avoid sticking. Press each slice of cooked plantain - you need to use a decent amount of pressure - and set aside. Repeat with the other slices.
Once all of the slices are flattened, re-heat the oil to medium-high (slightly warmer than the first time is best) and if needed, top up the oil a little. Prepare more kitchen paper on a plate for draining. When it's relatively hot, add a small slice - it should bubble a little around the edges straight away, if not then hold off adding more until it does. If it's hot enough, add a few slices in a single layer, taking care not to overcrowd them. Cook for around 2 - 3 minutes each side until golden and crisp, using a slotted spoon to carefully turn. Once ready, remove with a slotted spoon and drain on the kitchen paper.
Repeat to cook additional batches then serve, sprinkled with a little salt, while still warm. Feel free to add sauces etc for serving as you prefer (here I served with guacamole).