So now, remove from the torso and neck the end of the floss you had anchored there, and anchor instead what's left of your 4' end of floss by wrapping it around her torso and neck several times. Then take all the rest of the floss and unwind it into one long strand.

(If you come up with a better way, please let me know!)
One of your fins will be behind the other fin. That is the fin you want to weave first. So take your floss and wrap it down to the tip of the fin. (That wrapping doesn't have to be close together, just kept snug as you go.)
At the tip, start weaving in a figure eight. Go around one side, through the fin, around the other side, through the fin, on and on and on until your fin is done. It's important that in wrapping the fin you don't have floss going up the outside of the tail because it will mess up the weaving of the tail. Trust me on this one.

Then wrap to the tip of the second fin.
Starting at the tip of the second fin, weave it just like you did the first. When you get to the crossed over point it gets trickier and I put the floss on a needle to get in and out of cramped spaces. Once you are past that and on to the tail, it speeds up again, so take heart!
On Mermaid #1, the slender one, I had no problems with the floss on the tail. With #2 and her curves, I had trouble towards the end as the tail narrowed again. So I undid the weaving on the entire tail and did it again, this time adding tiny dots of glue to the outside edge of the tail on each side with each wrap. Time consuming but worth it as I know that floss isn't moving anywhere.
When the tail is done stop there (as with #2) or add some tail color to her waist (as in #1). Then tie the two ends of floss together and put a bit of glue on your knot before trimming.
All that leaves is a bead head and hair!
IF your tail doesn't work the first time, don't give up! On #1 I did the tail AND fins three times before I got it.
I hope the directions make sense.
Please share pictures if you make one. My daughter wants one in her stocking, and my son has requested a merman, so I think I might as well make a baby while I'm at it and they can have a whole family between them.
Rhea