After sewing on the buttons, flipped the piece over and sewed a tentacle shape with the other, blue, side of material. Turned it inside out for a tentacle sleeve.
Stuffed lightly with batting--just enough to give it a basic shape.
Next, stuck the wire up through to the tip (with a curved piece at the end so the wire wouldn't pierce through)
Fitting it onto the dress will be slightly more complicated--and I'm considering a belt kind of thing--just because it makes more sense for it to be easily taken off.