For this Little Mermaid costume, I took apart a baby onsie to use as a pattern to make the top. I added the seashell bra and stuffed it a little then added iridescent material to make them shiny. I bought fabric for the tail and took a pair of leggings and crossed one leg over the other and traced them to make a pattern out of newspaper and cut out the fin and tail and sewed it all together.
I hand drew the the long tail on newspaper to use as a pattern then cut it out of the material and attached it at the bottom.
That completes the construction detail my inspiration came from looking at Ariel photos from The Little Mermaid.