Take a small piece of white Ric-Rack ribbon and paint with regular fabric paint. I choose blue and yellow for contrast.
After the paint has dried, fix it with a hot iron.
Fold the ribbon in half, so that the white (unpainted) side is facing out, and add a few small stitches (according to the red line in the picture - I only did them with red thread to show the contrast for the picture).
Then open the Ric-Rack ribbon, so that the colored side faces out, press it down and stitch on a small eye.
I then sewed small fin like stitches on to the ribbon.
You can add an eye with a bead or do any other stitches for fin. You can even stitch on another tail.