Like ESNY said, ripe plantains are for maduros and green ones are for tostones.
For maduros it is best to choose plantains that are not totally black. At that point they are way to soft and will fall apart and burn. I like mine to still show a little yellow with a decent amount of black still on them. If they are too yellow they won't sweet enough, but that's a personal preference.
To prepare them just slice off each end and make a very shallow cut in the skin running along the length of the plantain and the skin will peel right off.
Next I cut them in half and then slice each half lengthwise into several slices and then fry them in hot oil until golden making sure to turn them so they don't burn. Keep in mind that the sugar content increases as they ripen and are therefore more likely to burn. I then drain them on paper towels. They are not usually salted. Also, don't use oilve oil. I use canola oil.
For tostones just select some nice size green plantains with no hint of yellow. These are trickier to peel as their skin is much thicker than the ripe ones.
My technique to peel is the following:
with a sharp paring knife, score the skin of the plantain along one of the little ribs ir seams all the way down without penetrating into the flesh. Depending on how big the plantain is you may need to score along two sides. Then I place them under some hot running water for a few seconds (don't know why, but my Mother always does it this way). Then take the edge of your knife and you should be able to peel away the skin in a large piece. Do not try to peel it as you would a potato. It will NOT work. You also want to cut off the ends.
Next take your plantain and slice it into little rounds about 1/2 to 3/4 of an inch thick (any thicker and they will burn before they are cooked in the middle. Then heat some vegatable oil over medium heat and fry the little rounds until they are golden....now, my technique is to fry them at a lower temperature at this stage so that they will cook through on the inside and still have a nice golden color on the outside. Once you have fried them up, drain them on a paper towel.
To smash them I use a tool called a tostonera which is just two peices of falt wood joined by a hinge, but you can use a cast iron skillet or any other heavy pan. Place a piece of plantain on a work surface and press down to smash it.
**A little note: many restaurants don't smash them very far and they end up rathe thick, most home cooks will smash them until they are failry thin and flat. This is a matter of personal preference, but I like mine thin because that's how we make them at home**
Once you have flattened them to your liking, fry them a secon time in oil that's at a slightly higher temperature than before.
Ideally, your plaintains should have been cook through from the first frying and so this one will just crisp them up. Drain them on paper towels, salt them and serve.
If you want to make these ahead, you can prepare them all they way up to the point where you smash them. You can hold them for quite some time before frying them the second time.
Are you interesting in making maduros (sweet, fried in chunks) or tostones (smashed, flattened pieces)? Both are originally cut into about inch thick pieces, but maduros uses ripened plaintains and tostones uses unriped. After the initial shallow frying, the tostones are then smashed and fried a second time to crisp them up.