When you start seeing floating leaves of the banana plant, let it stay on the surface for 2-4 days, and clip off the stem approx. 4 inches below the leaf. Let the leaf stay floating with good lighting. New roots will form below the leaf at the cut.
Let the new root grow to a suitable size (floating still), cut off the floating leaf, and plant the new root with a stub of old stem. My long long ago experiences areif propagating this way, I do not see "banana", but the new plant will grow like any other banana plants.
I've heard some that their banana plant sent out runners at one of the leaf's node, and new "banana" formed at the node in mid water. The stem below the banana will rot away itself and you got a new banana plant. But I've never had the patience to wait for that to happen naturally.
The thick banana is storage place for nutrients, like potatoes. I've never tried to break one off and see if it will grow into a new plant.