Pull the underripe bananas apart and place them, unpeeled on a baking sheet, large enough to fit the number of bananas that you want to ripen.
Place bananas in preheated oven and bake for 10-15 minutes, or longer until they reach the desired "ripeness" or they have brown spots, and/or become brown bananas.
Remove from oven, let cool and remove banana peels and use in your favorite banana recipes, like this banana bread.
Notes
You can use parchment paper or aluminum foil on your baking sheet in case the banana pushes out from the peel during baking, to keep clean-up a breeze.
After about 10 minutes, check your bananas to see if they have reached the desired ripeness. Some prefer only brown spots, some prefer them to be completely brown. I've found that brown spots are good enough for the recipes I make but you can definitely bake them for 15-20 minutes or longer if desired.