Launching a side hustle doesn't have to be complicated if you know where to start.

More people than ever are starting side hustles, data shows. And while you may — or may not — reach six-figure success, you can do specific things to ensure your second stream of income is worth the time commitment.

That's according to a group of experienced side hustlers who have done it themselves. "I think you can be successful with more side hustles than you think," Cody Berman, who co-runs online printables course Gold City Ventures, told CNBC Make It in January. "I honestly think people [who succeed] don't give up."

Here are four pieces of advice, from people who started side hustles that bring in at least $100,000 in revenue per year:

Find a gap in the market

Your first step to starting a side hustle is always figuring out what you can monetize. Then, you might want to research how your product or service would solve a problem or satisfy customers' needs.

High school senior Bella Lin — whose side hustle brings in six figures annually selling guinea pig cages on Amazon — learned that lesson as a pre-teen. In seventh grade, she started selling leggings for teen girls who couldn't afford larger, trendy brands. It brought in roughly $300,000 in revenue per year at its peak, but was never profitable.

She knew a market existed, because she was part of her company TLeggings' demographic. But she didn't fully realize how competitive the market was, and how challenging it would be to crack into it, she told Make It in January.

A lifelong guinea pig owner, Lin switched to a new idea: designing enclosures that were easier to clean and allowed the pets to roam more freely than traditional cages. That side hustle, called GuineaLofts, brought in $410,000 last year, according to documents reviewed by Make It.

Roughly 25% of that revenue was profit, Lin estimated.

"With TLeggings, I realized there were a lot of other companies trying to do [the same thing] ... With GuineaLofts, I [found] a gap in the market that wasn't getting much attention," she said.

Keep researching your market over time

Don't stop researching once you've launched your business. Your ability to stay up-to-date on industry trends and audience demographics can affect your earnings over time, Etsy seller Tim Riegel told Make It last year.

Riegel started his side hustle — making and selling steel fire pits — in 2021. The products quickly gained an audience, helping him make $169,000 on Etsy in a single year, according to documents reviewed by Make It.

But in early 2023, Riegel noticed his profits dipping. Research revealed a simple explanation, he said: Recyclable steel, gas and packaging supplies were becoming more expensive due to inflation, taking a chunk of his profits.

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

He studied his competitors' pricing, and decided he could afford to raise the price of his 275-pound fire pits to $950, up from $650 in fall 2022. He added new features and colors to justify the increase to customers, and his profit margins returned to 35% to 40%.

His takeaway: Evaluate prices on a monthly basis, so you can gradually raise them, instead of jolting your customers with larger, more sporadic increases.

"In today's climate, you have to really watch the costs of everything ... to make sure you're paying yourself," said Riegel. "If you're not making a profit, it's not worth spending the extra [time on your side hustle]."

Use your strengths and past experiences

Don't overthink it: If you're good at something, you may be able to get other people to pay for your expertise.

Just ask Becky Powell, a kindergarten teacher in Beaverton, Oregon. Powell brought in $125,500 in 2022 by selling worksheets on the educational resource platform Teachers Pay Teachers, according to documents reviewed by Make It.

As a teacher, Powell's specialty is teaching kids how to sight read, she told Make It last year. So instead of creating more generalized kindergarten worksheets — something plenty of teachers do — she created resources specifically for other educators to teach sight reading, too.

Her success came from creating something that stood out from a crowded marketplace, she said.

"My husband [a computer engineer] always told me, 'The riches are found in niches,'" said Powell. "For me [that meant] thinking about hands-on activities ... that would engage [students]."

Write, and stick to, a thesis

Once you've decided on a side hustle, write down a mission statement — and make sure every aspect of your business ties back into it.

Last year, Morgan Eckroth had three jobs, and all of them revolved around coffee. She had a $45,000 salary doing marketing for coffee brand Onyx Labs, worked as a barista in Portland, Oregon, and posted videos about coffee to TikTok, Instagram and YouTube.

Her social media accounts alone accounted for $96,000 in profit in 2022, according to documents reviewed by Make It, with most of that money coming from ads, brand deals and merchandise sales. It only worked, Eckroth said, because her videos always connected back to her goal of translating the warmth of a coffee shop to her followers.

"I think you need to have a thesis for why you're making content. It's great if that's 'I want to sell more of my product,'" said Eckroth. "Having that written out for yourself is really helpful, because then you can align every video against that thesis. I always ask myself, 'Am I accomplishing what I set out to on here?'"

For National Small Business Week, join CNBC's free Small Business Playbook virtual event on May 2. Experts and entrepreneurs will discuss the future of small business in America and share strategies to overcome obstacles and achieve success. Register here today.

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.