By the time Renée Zellweger arrived in London to begin filming “Judy,” she had spent a year learning how to sing like the film’s subject, the incomparable vocalist and entertainer Judy Garland.

And then she got the fake teeth.

“It was difficult for her to pronounce words, let alone sing,” Zellweger’s vocal coach, Eric Vetro, tells The Post. But after a few frantic FaceTime sessions with her Los Angeles-based singing tutor, she was ready to soldier on.

“Renée is just one of those people who sails above [such obstacles],” says Vetro. Plus, he adds, “it may have been difficult to have the teeth in, but in the long run [they] helped her sound more like Judy.”

Zellweger had to overcome a lot of obstacles to play a latter-day Garland in “Judy,” out Friday. For starters, the blond, squinty, blue-eyed “Bridget Jones” actress looks almost nothing like the frail, brown-haired, big-eyed Garland, who died in 1969, at the age of 47. And while the Texas native had sung before — her work as Roxie in the musical “Chicago” netted her an Oscar nod — she didn’t have the “Somewhere Over the Rainbow” singer’s singular set of pipes.

Even so, she succeeded in capturing the essence of that legend. After the film’s premiere at the Toronto International Film Festival, she received a three-minute standing ovation, and insiders are already whispering that she’s a shoo-in for best actress come the Oscars in 2020.

Here’s a look at what it took for Renée to become Judy.

The Look

“Judy” takes place near the end of Garland’s life, when — broken-down and broke — she goes to London to perform a memorable set of shows at the Talk of the Town theater in 1968.

Though Zellweger, 50, was close to Garland’s age at the time the film takes place, she looked about 20 years younger.

“Renée looks after herself very well … and Judy didn’t,” says Jeremy Woodhead, the film’s makeup and hair designer. “Alcohol and everything else in [Garland’s] life had taken its toll. Her face was much more gaunt and more haggard. So my brief was to ravage Renée a bit.”

‘We had to create the body of a broken woman — which is not Renée at all.’

At first, Woodhead experimented with prosthetics, only to chuck most of them because they got in the way of Zellweger’s performance, obscuring her subtle facial expressions. Instead, he used makeup to create the appearance of cheek jowls, bags under the eyes and lines in the face. (Brown contact lenses and a short brunette wig did a lot of the heavy lifting.)

He did, however, use a prosthetic at the tip of Zellweger’s nose, to give it Garland’s upturned silhouette. And he commissioned a pair of false teeth from Fangs FX —the same firm that made Freddie Mercury teeth for Rami Malek — for the actress to don on set.

“If you look at pictures of Judy near the end of her life, her teeth are quite gnarly,” says Woodhead. When the falsies first came in, they were so big that they overwhelmed Zellweger’s small mouth and oval face. “So I filed them back and back and back until they didn’t look distracting.”

Costume designer Jany Temime tells The Post that even the clothes Zellweger wears in the film helped the actress look and move like someone much older and frailer.

“We had to create the body, the body of a broken woman — which is not Renée at all,” Temime says, describing the way she would add rounded shoulders to Zellweger’s dresses so the actress would look more hunched-over.

“She was playing Judy by sort of arching her back and slightly twisting her spine. When she would come for her fittings, she would stand like that so we had to build the clothes with her in the Judy position,” Temime says, adding that she used a combination of vintage mid-century couture — including an Hermès scarf that belonged to the costumer’s mother — and clothes that she designed and built herself.

“I [tried to] think every detail that I could give [Zellweger] to get into Judy Garland’s skin,” she says.

The Voice

While costumes and makeup can do a lot to transform an actor into someone else, there was one thing Zellweger couldn’t fake: the singing.

Zellweger’s natural singing voice is much lighter and breathier than Garland’s full-throttle belting. And Garland’s songs are deceptively tough to sing, with lots of octave jumps and long, sustained notes. Plus, says vocal coach Vetro, director Rupert Goold wanted Zellweger to actually get up onstage and sing, for real.

“So not only was she taking on one of the most iconic voices in popular music,” Vetro says, “she had to do it live.”

‘Not only was she taking on one of the most iconic voices in popular music, she had to do it live.’

Instead of worrying about sounding like Garland right out of the gate, Vetro first worked on strengthening the actress’ voice. He had her relax her neck and jaw so she could get a more open sound, taught her breathing techniques so she could hold notes for a long time, and did exercises that would help with her intonation.

“After that, we started getting into what makes Judy’s voice so great and unusual and magical,” he says.

The duo listened to her recordings and watched her performances so they could analyze the shape of her mouth and the way she “manufactured her tone.” Vetro began asking Zellweger — who worked with dialect coaches Elizabeth Himelstein and Brett Tyne on Garland’s speaking voice — to do her lessons in character as Judy.

“She would improvise, come up with things,” says Vetro. One time, when they were doing an exercise that produced a nasal sound, Zellweger stopped and said, channeling Garland, “Wow, that’s not very pleasant — people aren’t going to enjoy that!”

“At that moment, I thought, ‘She’s really becoming her,’” says Vetro.

Himelstein, who helped unearth rare recordings of Garland speaking unrehearsed and unscripted, would have Zellweger listen along to tapes with a transcript, not only to get the late performer’s speech patterns and elocution, but also tap into the irony and humor in her voice.

“She was quite funny, you hear the banter when she would go on talk shows later in life,” Himelstein tells The Post. “And Renée absolutely captured all of that.”

“When you work with a great artist, the best thing you can do is stay out of their way and let them explore … get them to tap into the sounds,” Himelstein says. “And I knew Renee has the most brilliant ear, from ‘Bridget Jones’s Diary’ to ‘Cold Mountain,’ so she was really able to listen and then move onto the next level.”

But in the end, Zellweger’s transformation wasn’t just a matter of imitating her voice and gestures.

“She really captured [Garland’s] spirit and charisma — her empathy, emotion, heart, all of that,” Vetro says. “Watching the movie screening, even I forgot it was Renée.”