Last year, it became Humberto Leon’s job to shape the appearances of 20 young women, whose ages ranged from 14 to 21.

He decided what kind of clothing, shoes and jewelry they would wear. He told them how their hair should be cut and their makeup applied.

“You have to imagine, with 20 girls, I want each and every one of them to stand out,” Mr. Leon said.

Still, young women do not always take kindly to being told how to dress. There were tears. “That’s not how I like to do my hair,” some of them told Mr. Leon.

“I said, ‘I know, but trust me. I’m helping you own your personality,’” Mr. Leon recalled. “They think they know what’s best for them. And I have to give them an objective opinion of what I think would look great on them.”

Professionally, it was in their best interest to listen to Mr. Leon. Under his guidance, they could become the main characters in their own makeover montage — a tradition stretching from “Pygmalion” to “The Princess Diaries” to, perhaps more relevant to this group, “The Hunger Games.”

These 20 girls were in direct competition with one another. Six of them would eventually be named members of a new pop group. Upon its debut, this group would already have the support of Hybe, the company that brought K-pop to the world, and Universal Music Group, the world’s largest record company. The competition would also be the subject of a Netflix documentary series.

Throughout it all, the contestants’ public image would be in the hands of Mr. Leon, a 48-year-old fashion designer who rose to prominence in downtown New York during the 2000s with his store Opening Ceremony — a popular boutique for up-and-coming labels — then was recruited to reinvigorate a luxury brand in Paris, then started opening restaurants with his family during the pandemic in Los Angeles.

In September 2022, he was brought on as creative director for this girl group competition — a partnership between Hybe and Geffen Records, which is owned by Universal Music Group — in which 120,000 applicants from around the world were narrowed down to 20 contestants, or “trainees,” all of whom were relocated to Los Angeles to train intensively in singing and dancing.

When those contestants were announced in August, Mr. Leon dressed them for their first group photo shoot in matching gray schoolgirl uniforms. They wore blazers bearing the name of their competition: Dream Academy.

By November, half of this group was eliminated through a combination of fan voting and judges’ evaluations. The culling was chronicled on YouTube. (“We’re not forming a friend group, we’re forming a girl group,” one young woman said during a particularly tense elimination round.)

For the final photo shoot before the six winners were announced, Mr. Leon dressed the trainees now as “elevated” schoolgirls. This time they showed more skin in tailored gray sets, trading their chunky white socks for black mesh, looking like more polished, modern versions of Britney Spears in “ … Baby One More Time,” the music video that made a 16-year-old girl a star.

One morning in Hollywood, I watched as Mr. Leon oversaw these final portraits. He reminded one 17-year-old contestant, Megan, to correct her stance. She had a tendency to stand with her legs wide apart, which Mr. Leon had nicknamed “the Megan.” As in, “Don’t do ‘the Megan,’ Megan.”

Later, while the 10 remaining trainees filmed a music video, I noticed that Megan had a way of staring down the camera with a cool, come-hither expression — similar to the seductive one Ms. Spears adopted. (Megan, of course, was not yet born when “ … Baby One More Time” was released.)

This tendency was not corrected.

When it came to being sexy, Mr. Leon said he had always told the girls, “Whatever you’re doing, do it for yourself, because you want to feel that way.”

K-pop, But Not

“Dream Academy” was not Mr. Leon’s first time working with a girl group.

In 2021, he met the Linda Lindas, a punk quartet that went viral after performing at the Los Angeles Public Library. At the time, its members were between 10 and 16 years old. They had come to eat at Mr. Leon’s restaurant Chifa, named for a Chinese restaurant his mother, Wendy, opened in Peru in the 1970s before the family moved to the United States.

When Mr. Leon offered to direct their first music video, the group said yes. “Growing Up” showed the four girls and four cats shredding in a suburban home, dressed in 1970s-inspired outfits.

When she saw the video, Michelle An, now president of creative strategy at Interscope Geffen A&M, said she thought it was “so cute and so innovative and so appropriate for their age.” She was particularly taken with the illustrations of cats painted on the girls’ closed eyelids.

Ms. An’s job is to help her labels’ artists, like Billie Eilish, with “visual world building,” she said. “You make this music — what imagery do you want out there to help your fans understand what this song is trying to say?”

Geffen had an unusual project in the works with Hybe, a Korean entertainment powerhouse. What began as a conversation about music distribution ended with Bang Si-hyuk, the chairman of Hybe, proposing that they build a group together. Hybe would bring elements of K-pop’s famously rigorous training and development program — the same system with which Hybe built BTS — to the United States for the first time, filling it with trainees from various regions, not just East Asia.

One hurdle, though, was the Americans’ concern that the group could seem too factory-produced. “K-pop has a reputation of being manufactured,” Ms. An said.

Even outside K-pop, the history of boy bands and girl groups reeks of “not being as organic and real,” said John Janick, the chief executive of Interscope Geffen A&M, pointing to glossy reality shows of the 2000s, like “Making the Band.”

In order to make the group feel real, the executives said, the girls had to feel real. Their personalities couldn’t be forced; there would be no extreme archetypes, no Posh or Sporty or Baby Spice. They needed someone who could draw out the girls’ distinct backgrounds and abilities but also make them cohere visually as a group. They were convinced Mr. Leon could be that person.

“In the entertainment business,” Mr. Janick said, “everybody wants to have taste, but not all people do.”

‘A Curious Mind’

Instead of going to fashion school, Mr. Leon likes to say, he worked at the Gap for 10 years.

At 14, he was hired at a store in West Covina, Calif., and learned he had a skill for designing windows. He continued working on visual displays for Gap while attending the University of California, Berkeley. After graduating in 1997, he accepted a corporate job with Old Navy in San Francisco.

In 2000, Mr. Leon left for New York, working at Burberry as the director of visual merchandising.

In 2002, he founded Opening Ceremony with Carol Lim, a college friend.

“We have a similar approach to life,” said Ms. Lim, who was the business-minded chief executive to Mr. Leon’s creative director. “A curious mind,” she called him.

A decade later, having cultivated a Midas-touch reputation for coolness, the duo became design directors at Kenzo, a LVMH-owned brand in Paris.

At Kenzo, Mr. Leon took a particular interest in marketing visuals. Mr. Bang, the Hybe chairman, called a 2016 fragrance advertisement starring a frenetic dancing Margaret Qualley, directed by Spike Jonze, one of his “favorite fashion artworks.”

Mr. Leon and Ms. Lim left Kenzo in 2019, then sold Opening Ceremony and closed its stores in 2020, moving to the same neighborhood in Los Angeles to raise their families.

Around this time, Mr. Leon said he had an epiphany: Even if he was “good” at it, he didn’t have to keep working in fashion. “I was able to create a feeling, and a feeling can transfer,” he said. “I decided to open up my world a bit.”

Sometimes Mr. Leon still designs clothes; recently he got a call from the choreographer Justin Peck about creating costumes for a spring performance of the New York City Ballet. But what appeals to him now is making things not for runways but for culture. For example, when Heidi Bivens, the costume designer for “Euphoria,” was working on the teen drama’s first season, she sourced several outfits from Opening Ceremony. The “‘Euphoria’ effect” became a phenomenon, inspiring trends in fashion and beauty.

“I went to them, and I said, ‘For Season 2, let’s design this from scratch, so everything you see on ‘Euphoria’ is something we’ve never seen before,’” Mr. Leon said. Consumers could then directly buy the clothes they saw onscreen.

That pitch didn’t work out, but it’s an idea Mr. Leon still wants to explore. .

‘Trust Me’

In November, Mr. Leon showed me a video of his twin daughters at their 10th birthday sleepover. In matching pajamas, the girls recreated choreography from a “Dream Academy” mission. (Missions were essentially live music videos in which the trainees’ singing and dancing skills were tested.) Five of the trainees had participated in a rump-shaking cover of “Buttons” by the Pussycat Dolls.

The twins had become invested in who would win the competition. So had fans around the world, some of whom paid for billboards in an effort to drum up votes for their favorites, like Sophia (20, Filipina) and Manon (21, Swiss-Ghanaian).

Still, during the 12 weeks that the competition unfolded on YouTube, “Dream Academy” did not exactly become an international phenomenon. Just three of the trainees’ 15 missions cracked more than one million views — somewhat underwhelming by K-pop viewership standards.

Next year, around the time the six winners will release music under their new name, Katseye, the project has another chance to break through. In summer 2024, Netflix will release a documentary series about the competition by Nadia Hallgren, who directed the Michelle Obama documentary “Becoming.” This may be the ideal format for capturing the drama, major and minor, of the process.

Within just an hour on set, I watched a trainee in a silver paillette minidress with tendinitis in her knees fight back tears, take after take, while filming a video for an original song called “Dirty Water.” I watched another in a tube top and reflective wide-leg pants be told to exert better control over her hair flipping.

I also watched the adults in the room engage in a delicate dance of evaluating, correcting and handling these young women, while trying to be sensitive to the fact that they were young women. (The youngest was 15.)

“Tell the girls it’s us, it’s not them,” the director of one music video instructed an assistant during a technical delay.

In hiring Mr. Leon for the project, Ms. An hoped his experience raising two girls would help in this regard. His first self-appointed task was interviewing each contestant individually before making any decisions on their new looks.

“I wanted to look in their eyes,” Mr. Leon said. “I wanted to ask them the hard questions about their upbringing.”

He told the trainees who came in wearing heavy makeup to take it off. “I want you to look gorgeous and beautiful, and I want you to be yourself,” Mr. Leon recalled saying.

“I think it’s hard for people to see themselves,” Mr. Leon continued. “You need somebody to tell you that you look amazing without much.”

To assist in the makeovers, he brought in stylists who worked on the “The Idol” — an HBO show about the relationship between a pop star and a cult leader. He brought in the hairstylist to Bella Hadid.

To the 14 trainees who didn’t make the final group, he seemed to want to send a message: “I did the best thing I could for you. And you have to trust me.”