how do I know when plantains are right to be cooked ( to make tostones)?

I love the suggestion of pressing them before frying!

For tostones, you want the plantains to be still very underripe, with green to barely-turning-yellow skin. They should be starchy enough to get very crisp when you fry them. Cut rounds of plantain about one inch thick, then place them between two sheet of plastic wrap or parchment (or a tortilla press) and crush them under a flat skillet to flatten. Then fry, fry away!