Even billionaires think about starting side hustles.

If Mark Cuban were 16 years old again and "needed to make some extra money," he'd start one specific side hustle in just three steps, he tells CNBC Make It.

First, he'd learn how to write prompts for artificial intelligence language models like OpenAI's ChatGPT or Google's Gemini. Next, he'd teach his friends how to use those prompts on their school papers. "Then, I would go to businesses, particularly small- to medium-sized businesses that don't understand AI yet," says Cuban. "Doesn't matter if I'm 16, I'd be teaching them as well."

More than half of Gen Zers in the U.S. currently have side hustles, a LendingTree report found in February. AI prompt engineering — or, the ability to phrase inquiries to chatbots to get desired responses — can be a particularly lucrative opportunity. The average pay for AI tutors starts at about $30,000 per year, and full-time AI prompt engineers can make up to $129,500, according to job board platform ZipRecruiter.

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You don't need a college degree to become an AI prompt engineer, but you do need practice — and, often, certifications — to learn how those large language models operate. Some online certification courses, like Vanderbilt University or IBM's offerings on Coursera, say you can master the basics in one month.

Cuban's hypothetical side hustle is more high-tech than his actual first job, selling garbage bags door-to-door to his neighbors outside of Pittsburgh at age 12 to save up for a new pair of basketball shoes. He continued to earn extra cash as a teenager by selling collectibles like baseball cards, stamps, and coins, eventually helping him pay to attend Indiana University. There, he bartended, hosted parties with cover charges and even picked up work as a dance instructor.

After a brief post-college stint in banking, Cuban turned to entrepreneurship full-time. He sold his first company, a software startup called MicroSolutions, to CompuServe for $6 million in 1990. His second company, audio streaming service Broadcast.com, made him a billionaire when he sold it to Yahoo for $5.7 billion in 1999.

Today, Cuban has a net worth of $5.7 billion, according to a Forbes estimate. He spends much of his time advocating for his online pharmacy Cost Plus Drugs, which aims to make a variety of common prescription drugs more affordable by selling them at cost, plus a 15% markup.

 "I was a hustler ... I have always been selling," he said during an episode of ABC's "Shark Tank" that aired in 2016. "I always had something going on. That was just my nature."

Correction: This story has been updated to reflect that Mark Cuban sold garbage bags door-to-door at age 12.

Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank," which features Mark Cuban as a panelist.

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