I have tried it a few times with the follow advice:
Chill (over night) or freeze the banana(s) first (I prefer frozen when possible).
Before inserting with peel on wash with soap, water and dry.
Use plenty of lubricant first and during insertion of unpeeled banana to loosen anal sphinters.
Peel banana after initial placement and reinsert whole (if possible).
Where possible use a long banana, curved (for prostate pressure) when in place.
Using one banana allows for the effect of an unbroken banana (further penetration of the bowel).
I am able to hold the one banana for over an hour (allowing for thawing) before needing to fill my diaper.
Usually have bladder release while expelling the banana. Enjoy the experience if you are so inclined. Share with your significant other. O:-)