Even though bananas are relatively cheap for a fruit there might be better fertilizers. If you're talking about just using the skins then it's probably just best to use them as part of a mulch.

Years ago I used my Cuisinart blender for mixing up food scraps to use in the garden. A banana peel bound up the cutting blades and nearly burned the motor out.

Why would you want to?

It is just one of those garden fads that pops up now and then and then disappears once a few folks try it and find it doesn't do anything.

It certainly is not a recommended fertilizer for anything. It provides nothing that isn't better available from many other sources.

Dave