Using a side piece from the body of the sweater, I cut a diagonal line across. I loosely measured how many inches between my son's behind and the floor. This is how I decided on the tail-length.
I cut more of those fins, gradually going smaller as I got closer to the end of the tail.
I tucked the fins inside and pinned where my seam would be. I stitched it on my machine, turned it right-side-out, then used poly-fil to stuff it. Looks great!