Stretchy swimsuit liner material was used to create the upper body to look like bare skin. Attached is a bra like swimsuit top, sewn to look like shells and padded with quilt batting. The long tail also has quilt batting in it at the bottom and in the pink tail fin. About half way down the tail there is a slit up the back making the top half of the the long tail more like a skirt so she can walk, then the rest of the tail can be swooped up and carried on one arm off to the side. (It has a strap attactched for her to carry it around.) The only thing in the costume that wasn't hand sewn was the reddish/orange wig.
No pattern! I just had my daughter lay down on the reverse side of some wrapping paper, drew around her and the cut out the shape and created my own pattern.
The tail was simply sewing a tube of material as it tapered to the end, attaching the fin that I had sewn separately.
I hand-sewed the little bra cups to look like shell shapes, and then attached them to the skin-like shirt (a basic long sleeve shirt to keep her warm and to look like it was her skin).
The wig was a real Disney Little Mermaid wig bought at a costume shop.