To bring Disney’s iconic mermaid princess Ariel to life in “The Little Mermaid,” costume designer Colleen Atwood constructed a life-sized tail that went from star Halle Bailey’s chest all the way down past her legs.

“We made it to scale and 3D silk-screened the tail and painted onto that so you could get the nuance of the colors,” Atwood says. “We used different layers of sheer material, which gave the tail and scales an iridescent effect.”

Her biggest challenge was blending the tail’s scales into Bailey’s skin. “I solved that by putting little fins made of fabric so there was a delineation between where the fish ended and the skin began,” she says.

The intricate detail helped the visual effects team, who then scanned the full costume to develop the CGI for the underwater scenes.

For Ariel’s dress that she wears when she’s finally on land,...