One day, in 2016, prima ballerina Sara Mearns received a call from the New York City Ballet’s costume shop saying she had to stop by and meet their new intern.

Imagine her surprise when she arrived at the sewing room and saw Oscar-winning actor Daniel Day-Lewis diligently stitching among the bustling seamstresses and cutters working in the shop.

“She was just in disbelief and total surprise,” says Marc Happel, director of the New York City Ballet’s atelier, who taught Day-Lewis how to drape, sew and measure for the actor’s role in his latest — and ostensibly last — film, “Phantom Thread,” which debuted on Christmas Day.

“In the beginning, I think people were a little surprised to see him sitting at the sewing machine or walking in the door,” Happel tells The Post. “But he moved in quite easily and became quite friendly with the entire shop.”

Day-Lewis is known for his immersive approach to acting: He famously built his character’s 17th-century-era house with his bare hands for “The Crucible,” and trained as a butcher for his role in “Gangs of New York.”

In “Phantom Thread,” he plays Reynolds Woodcock, a fictional 1950s London couturier. Not only does the film include several scenes showing Woodcock practicing his craft, but director Paul Thomas Anderson wanted Day-Lewis to have some input in the fabrics and colors of the gowns his character would design.

So the film’s costume designer, Mark Bridges, decided to put the actor through fashion boot camp with Happel.

The yearlong process, which began in late 2015, was grueling.

“It started so slow that sometimes we would work day after day after day just picking up pins and pinning them into a piece of fabric,” says Happel. Often, the pair would spend hours just slicing pieces of cloth.

“There’s a certain way in which people cutting fabric would handle scissors, and many times it can be a real giveaway if you don’t [do it correctly],” says Happel.

Then Happel had Day-Lewis spend two weeks taking measurements of whoever happened to be in the shop — a skill the actor shows off in the film during his character’s thrillingly intimate first fitting with his future wife, Alma, played by newcomer Vicky Krieps.

‘We would work day after day after day just picking up pins and pinning them into a piece of fabric.’

The scene impressed Bridges. “The whole sort of studying it, stepping back, looking, turning your subject into the mirror so you can look at it in a reflection,” he tells The Post of Day-Lewis’ performance. “Even down to the looking-over-the-glasses thing that I do … he got it absolutely right.”

After hours, Day-Lewis would study fashion books given to him by Bridges and Happel, and practice hand-stitching fabric swatches to show to his teacher the next day. The actor even toured the Costume Institute at the Metropolitan Museum of Art, where the textile experts showed him masterpieces by the likes of Dior and Balenciaga.

By the time Day-Lewis went to London to begin filming, he not only had hand-embroidered costumes for the New York City Ballet’s production of “Firebird,” he had constructed a copy of a Balenciaga dress from scratch, based on photographs, with just a little bit of help from Happel.

“It came out really, really well,” says Bridges, who inspected the gray flannel dress lined with lilac silk himself. “I think his wife, [Rebecca Miller], has worn it out in public.”

Even Cecile Van Dijk, head cutter for the costumes in “Phantom Thread,” was impressed. “He was really such a natural at it,” says Van Dijk, who appears in the film and was asked to give Day-Lewis refreshers and tips before his fashion-making scenes. “I really think I was just there for him as a confidence-booster, but I didn’t think he really needed it that much.”