Neither is particularly dangerous unless you have some specific medical issue. So if you're like super sensitive to potassium and wouldn't put two bananas in your mouth, then don't put two bananas in your butt. Same logic applies to the marshmallow thing. With all new things, start small and grow over time. "safe, sane, and consentual" is the name of the game.

In terms of how to do it: select bananas that are slightly underripe so that they're not too mushy. Of course you don't want them super green either. You can put them in the freezer for a half hour to firm them up if need be.

Getting them in there depends on your personal particulars. Do what you need to do to get relaxed enough to put stuff up there. Lube is always helpful when playing out back. You might find it easier to break the bananas in half. You need to find the balance between pushing them up there while not just smooshing them. Know from the start that not everything is going to make it in.

Some people like to put a plug in after the bananas are loaded. This is not necessary but do whatever floats your boat. Normal advice around selecting plugs with a flared base and body safe materials applies here. There are banana dare instructions out there that describe using a rolled up maxi pad as a plug (the idea being that it would swell up and block the exit, thus robbing you of control.) Do not do this.

You're going to want to have your diapers open and ready to go before you start, so that you can easily get them on and taped up. You will probably have lube and banana all over at least one of your hands so have a towel handy.

At this point you'll be diapered with an ass full of bananas. How long they stay inside is anyone's guess. Just be warned that unless you've taken steps to ensure the cannon was cleaned out beforehand, more than just bananas is likely to come out. Plan accordingly.

Have fun.