"In April of 2015, I was going through the hardest time of my life. My dog Diva died, my relationship ended, and the last thing I wanted to do was write another sad song about it," the singer wrote in the video.
"Ant saw Harry on my lock screen and said: 'Let's write a song about him instead.' So we did.
"A week later, we made a music video. For $300. Directed, produced, edited by me. Then, we shelved it for six months. And wrote the rest of the record.
Karter said Harry's birthday was then fast approaching and it was time to release the song, but wondered how they were going to "cut through the noise to get noticed."
"After weeks of going back and forth, arguing, debating, crying and yelling [we asked ourselves] what would the rock legends that inspired me have done in 2019?
That's when the idea struck.
"Let's tattoo Harry Styles' face ... on my face," she wrote.
They then came up with a scheme to get away with the perfect stunt.
"To pull this off, we needed a celebrity tattoo artist to make it look real. So we called Romeo.
"He traced Harry's 2017 Rolling Stone cover, applied to my face, and then a special FX makeup artist did her thing."
"By the way we know it looked ridiculous, that was the point."
"We snapped that photo, shot some b-roll and prepared for your lives to change forever.
Karter is seen looking nervous before making the stunt go live.
"It was a Saturday night. I did a quick 10-minute meditation then, at 8.07pm it was go time.
When she woke up the next morning, she realised her plan was working.
"That photo was EVERYWHERE!," she exclaimed as photos of the media stories flash up in the video.
"But none of you knew what was coming next. At 9am on Monday, Billboard announced the release of my video and single Harry. And that's when things really took off."
"Every major publication and news outlet in the world covered the story and more importantly the song. I had your attention."
She then explained that she went into hiding for three days as TMZ was looking for her.
"My friends and family were freaking out. We got calls from talk shows, radio stations, YouTubers record labels and every other human being we've ever met. Everyone wanted to see my Harry tattoo in person."
She then reveals they attempted to put it back on, but it turned out worse than the previous one — and had trouble getting it off. So they then decided it was time for the world to know their secret.
"The tattoo was not real," the singer admitted.
At the end of the video, she shared a message to her all-time favourite star Harry Styles and the rest of her fans.
"I love that Harry has introduced rock to a new generation of fans. Plus he's nice to look," she wrote with a smile emoji at the end.
"This has always been about rock and roll. Harry is an anthem for the fans. The tattoo is gone. All that's left is the music. Which is was my rockstar team and I had planned all along."
