Ripen green bananas quickly by closing them in a paper or plastic bag with a pear, tomato or apple, or by baking them in a 170-degree oven and removing them before they form brown spots. Bagging takes about 24 hours. Baking requires an hour and then cooling in the refrigerator.
To quickly make already-yellow bananas overripe to use in recipes, leave the peels on and place them on a cookie sheet with a minimum of an inch between them, so the heat can circulate. Bake at 300 degrees for an hour, until the skins are black. Remove them from the oven and place them in the refrigerator until they are cool enough to peel and mash.