Here is a cute pattern for a Party Dress/Mermaid Costume. Fits Barbie Dolls.

I used D.K Sport weight. Bernat softee baby. You could easily use another brand of yarn with D.K. Sport Weight

You Will need:
3.00 mm hook
Yarn (2 colors)
needle
thread
sequins(optional)
1.25 inch elastic(for crown)
snap fastens (7 or 8) preferably plastic or close to same color of dress.