25-year-old spent $300 on a sewing machine in high school—now her seven-figure denim brand is worn by Taylor Swift

Elena Bonvicini didn't set out to start a multimillion-dollar denim brand worn by the likes of Taylor Swift, Gigi Hadid and Kendall Jenner. She just liked thrift shopping. 

Every summer as a teen, the Southern California native would go to Wisconsin to see her grandparents. She'd visit thrift stores in the area and rifle through the denim selection, searching for Levi's that she could fashion into cutoff shorts for herself and her friends.

Originally, it wasn't supposed to be a business. Bonvicini had fun making the cutoffs and, as she told CNBC Make It, "always loved having things that no one else had." But that changed when she was back at high school and someone asked about her shorts. 

"I got stopped by a girl who was two grades below me and she was like, 'Oh my gosh, where'd you get those shorts?'" Bonvicini, now 25, explains. "And I said 'Oh, I made them!' And she was like 'Can I buy a pair? Can you make me a pair?'" 

Bonvicini had never sold clothing before, so she relied on her best "guesstimation" to set the price for her first sale at $30. More girls started asking Bonvicini about the pants, and the young designer realized that she had a business on her hands. 

Bonvicini was soon selling hundreds of reworked vintage pants to classmates from her high school and other schools in the area. Every Friday, she would set up shop in her gym locker room and sell jeans for $10 each. 

"I would lay them all on the locker room floor and the girls would come and have a try-on party," she says. "This was before Depop or Poshmark, so it was kind of a new idea to be upcycling and buying clothes from the thrift store and making them cute."

Even charging her friends and classmates just $10 or $30 for a pair of jeans, Bonvicini turned a