One way to ripen bananas quickly is to put them in a paper bag and close the top. This traps ethylene gas and ripens the fruit faster. You can also cook bananas to soften them: for the oven, lay them unpeeled on a baking sheet and bake at 350°F until they're soft to the touch. Or, poke holes in the peels and microwave bananas until they're soft. Placing bananas in the fridge will also make them turn brown and soft, but this may take a couple of days.