If you have more time, place the bananas in a brown paper bag with an apple or an avocado, and let them there, at room temperature, for a couple of days. For a faster method, peel and place your unripe bananas on a baking sheet and bake in hot oven at 300°F (150°C) for about 15 minutes. As an alternative, microwave in 30-second intervals until brownish.