When there have been a few frosts your bananas or cannas will look scary!
Here's how to do it (the proceure is the same for both cannas and bananas):
Step 1: Move any mulch away from the base of the stems.
Step 2: Cut the stems off! It may seem drastic, but it'll be OK! Next spring the new leaves will emerge from the center, bigger than ever!
Step 3: Cover the stems with a few inches of mulch. Mix in some leaf litter so that there is some air circulation. The stems will weep fluid for a few days that could cause rot if there was no air circulation.
Step 4: Uncover in late spring! You'll probably see a little green starting to poke up through the middle of the stems you cut off!