Tostones are a delicious fried snack found in just about every restaraunt on the island. Consisting of a green plantain (banana) that you fry, smash down into a chip and fry again - it's a quick snack that can go with almost any meal.

A preferably deep and thin pan for frying oil
A frying pan to smash your plantains, or for perfect tostones every time you can pick up a Tostonera

Peel your plantains, removing all of the green exterior and stringy material.
Cut your plantains DIAGONALLY. Having your plantains cut diagonally gives your chips more surface when we flatten them out. (This youtube video has a great example of a diagonal cut)
Once your oil is hot, place your chopped plantains and let them cook in the oil until they become a light golden-brown.
Remove your Tostones and place on a cutting board and using a pot, pan or your new Tostonera, smash the lightly fried plantains into thin (1/4 inch or so) plantain chips, the thinner they are the more crunchy they will get - so experiment and figure out what level of fluffy vs crispy you prefer!
Throw your chips back to fry in the oil for a few more minutes until a healthy golden yellow develops on both sides of the plantain chip
Throw them into a bowl with some napkins to blot off the excess oil
Sprinkle some salt (or what ever seasonings you prefer, experiment!) and enjoy thme with some puerto rican mayo-ketchup!

Tostones - Fried Green Plantain