Originally, I had the batting extending down the whole length of the tail into the fin, but this looked gigantic on my Boston Terrier, so I cut the linen and batting to end at her rump. Then the micro-fiber was able to just drape to the ground with some slack. Then I cut out a tail fin from batting, roughly in the shape of a trapezoid. To make the five “fingers” on the tail, I cut four triangle notches about 2inches deep into the long side of the batting. After that I placed it in between micro-fiber like I did for the flippers. I pinned tightly along the edges of the batting where the notches were, which emphasized the webbing in the tail. Then I sewed up to the top of the tail along each side of the cuts to make a small ridge, like cartilage, between the larger sections of the fin. See pictures.
I extended the micro-fiber a little past the batting so I would have some slack to fold it under ¼”, then another ½” or so. Then loosely gather the “tail” on the main body and pin that to the tail fin you just made. After this was sewn in place, I sewed the loose micro-fiber together to give the tail more continuity.