CEO created a side hustle in his college house—now it brings in $200 million a year: His No. 1 piece of advice

For anyone who aspires to start a business in their dorm room and grow it into a multimillion-dollar company — or simply use it to help cover tuition costs — Seth Berkowitz has some advice.

Berkowitz, the 43-year-old co-founder and CEO of Insomnia Cookies, got his start baking cookies in his college house at the University of Pennsylvania and delivering them around campus as late as 4 a.m. Today, he oversees more than 260 Insomnia Cookies locations that brought in more than $200 million last year, according to documents reviewed by CNBC Make It.

His advice is twofold. "One, get a mentor ... Find somebody who's done this before you, or done something similar," says Berkowitz. "Two, do it. [College] is such a great time to try something."

Here's why he recommends both, and his advice on picking your dorm room business idea.

'Get a mentor'

Berkowitz wishes he'd known this particular piece of advice sooner, he says, because "it's pretty lonely at times" to run your own business — and it can help to have someone to turn to for advice and reassurance.

That was especially true during Insomnia's "hardest years ever," in the wake of the 2008 financial crisis, he says. Berkowitz downsized the company's corporate staff and worked a lot more as a result, including driving between his home in New York and Insomnia locations in Philadelphia, Syracuse and beyond on a weekly basis.

A trusted mentor can help for a simpler reason, too: "You don't know what you don't know," says Berkowitz. In his case, he experimented with different business models for nearly a decade before finally figuring out how to turn Insomnia into a profitable business.

DON'T MISS: The ultimate guide to earning passive income online

"You might be arrogant, like I was at times, and try to push through and be comfortable with trial by fire," Berkowitz says. "But learning from those who've come before you really allows you to jump over some serious potholes."

Many people benefit from a reliable and experienced mentor, not just entrepreneurs. If you're a college student, your school might even be able to connect you with a professor or alumnus with relevant experience.

Just 'do it'

Berkowitz doesn't advise dropping out. "There's a lot to still gain from college," he says.

But most college students don't fully appreciate "how much time you have" to experiment with a variety of ideas and ventures, says Berkowitz.

"You are taking lots of classes and there's lots of work, but there's so much more time than you realize," he says. "It's a great opportunity to try something that's just a little bit less risky than what you're about to face a few years after you graduate."

Running a business — even just a side hustle — is definitely a lot of work, and you need to be willing and able to balance that with your course load and social life, he notes.

"It could be too much for some," Berkowitz says. "[But] if that's your personality, where you love the action, it's really a great fit."

How to pick your dorm room business

First, consider your own interests, says Berkowitz: He loved sweets and staying up late.

"It's so critical that you do something that you're passionate about," he says. "You don't want to spend your time working on something that's not interesting."

Second, embrace "a lean mentality." Rather than raising funding before you've generated a dollar of revenue, "people should start a little smaller, think it through, develop a business model, make sure there's sustainability and then go, and go fast," he says.

Berkowitz estimates he spent roughly $150 on baking ingredients and supplies before he started testing cookie deliveries around campus. The experiment earned him $10,000 of profit in his first semester, showing him the business idea was viable, he says.

Wrangling the costs of paying employees and renting real estate proved more challenging as Berkowitz grew the company. Still, he's glad he started small to prove that the concept worked before bringing on investors and opening brick-and-mortar stores, he says.

"If it didn't work, it wasn't a lot that I was going to give up," says Berkowitz.

Want to make extra money outside of your day job? Sign up for CNBC's new online course How to Earn Passive Income Online to learn about common passive income streams, tips to get started and real-life success stories. Register today and save 50% with discount code EARLYBIRD.

Plus, sign up for CNBC Make It's newsletter to get tips and tricks for success at work, with money and in life.