For bananas to ripen evenly, they should not be stored in plastic bags from the grocery store. Removing them from the plastic lets air and gases circulate, which is necessary for proper ripening.
For bananas that are still green or that haven't reached their peak ripeness, put them in a paper bag. The bag traps the gases released by the bananas, helping them to ripen more quickly. Place a single banana or a bunch in the bag.
To speed the ripening process, add a tomato or an apple to the bag. Both fruits add to the gases being released by the banana, causing the banana to ripen much quicker.
Cooler temperatures slow down ripening. Do not store bananas in the refrigerator unless they have reached their peak ripeness. Put the bag of bananas in a warm spot, such as the top of the refrigerator, to allow the bananas to stay at an optimal temperature to ripen them as fast as possible.