What's the technique for frying plantain?
In the Caribbean green plantains are treated much like potatoes.  One technique that is common is twice-frying, usually with a soak in water either before the first fry, in between, or both.  Soaking plantains allows them to absorb a little water to help steam the inside as it fries, much like one would do for American-style french fries (UK: chips).  One example of this technique is Tostones.  The Puerto Rican classic cookbook "Cocina Criolla" by Carmen Valldejuli calls for soaking the plantain pieces in room temperature water with salt and crushed garlic for 15 minutes as well as dipping them in water immediately before the second fry.  Like many potatoes, green plantains will often turn brown if left out in the air for too long after being peeled, soaking in water also helps prevent this from occurring as well as reducing the amount of free starch on the outside which can cause over-browning.

Elsewhere, both in the Americas and Africa, green plantains are cut very thinly and fried into plantain chips (UK: crisps).  These are usually 1-2mm thick and also benefit from presoaking.

If you decide not to fry them you could also boil them in salted water like a potato or boil them in vinegar with onions, garlic, and spices to make an escabeche. You could also partially cook them in salted water or stock then bake them in the oven.