You should harvest bananas when they are plump, but green. (Don't wait until they are yellow!). Once they are harvested hang them in a cool shady place, and then you can cut off hands one at a time and ripen them in a plastic bag with an apple (or any other fruit that gives off ethelyne gas). See info on the CRFG (California Rare Fruit Growers) site below.
www.crfg.org