So Far,

The 'body' of the mermaid tail blanket. I picked color and stitch pattern that remind me of tropical fish, like parrot fish. The top is to the left, and was knit flat. A little over halfway down, I joined the sides, began knitting in the round and making decreases. The bottom was bound off, then stitched together, making the lower part like a sack. 

I had several ideas on how to construct the tail, like so many pretty stones I'd pick one and turn it over and study it, and then a different one would catch my fancy. Finally, I settled on a look I wanted, and let the concept drive my design. Now I'm playing around with the construction.