This is the biggest money mistake millennials make, says author of 'You Are a Badass at Making Money'

This is the biggest mistake young people make when it comes to money
This is the biggest mistake young people make when it comes to money

Before Jen Sincero made a name for herself as a success coach and New York Times best-selling author, she was scraping by as a freelance writer, earning $28,000 a year. "I was living in Venice Beach, California, in a teeny, tiny, crappy apartment," the now 53-year-old tells CNBC Make It.

And, until eight years ago, she had nothing in savings.

Today, Sincero is "making seven figures as a success coach and author," she writes in her 2017 bestseller, "You Are a Badass at Making Money." And if she were to give millennials one piece of money advice, it would be the same advice she'd give her younger self: Focus on earning.

"The biggest money mistake that young people make today is not really focusing on making it," says Sincero.

That's a lesson she didn't learn until her 40s, she admits. Still, as soon as she started focusing on earning, her financial situation started to improve.

Jen Sincero, #1 New York Times bestselling author and success coach
Source: Jen Sincero

Once she decided to get serious about making money, she used that goal to help her make decisions about what projects to take on: "If it wasn't going to make me money, I didn't do it," Sincero says.

"I went about it the same way you would if you were losing weight," she continues. "You wouldn't fill your house with crappy, fattening food, you wouldn't hang out with lazy people — you'd get your butt to the gym, you'd buy workout gear, you'd eat healthy food, you'd hang out with people who are in that mode.

"So that's what I did: If if wasn't making me money, I wasn't interested in it. And I really worked hard and focused on all the things that I knew would increase my income."

You can't just work hard, either: You have to be smart and efficient. "I was working my butt off as a freelance writer making zero dollars," she says. "I make way more money now and I work way less, so that's a big fat lie that you've got to work hard all the time if you want to make a lot of money."

The key is to find "a viable income stream."

If it wasn't making me money, I wasn't interested in it.
Jen Sincero

Sincero stopped freelancing and set up an online business helping entrepreneurs write and sell book proposals. Within three months, "I tripled my income," she says. And that was just the beginning. Having experience informally coaching female entrepreneurs, Sincero started working as a "success coach" around 2008. Over the next several years, her business took off and she worked with hundreds of clients.

Her achievements as a coach led to speaking engagement opportunities and, eventually, a book deal.

Her first book, "You Are a Badass," was published in April 2013 and became a No. 1 New York Times best-seller. She has since published "You Are a Badass at Making Money" and "You Are a Badass Every Day." The three books, combined, have sold over five million copies.

Before you actually start generating more money, you have to believe you can make it happen, Sincero says. "Get into the emotional state of what it would feel like when you do make that money and get really excited about it and make it a non-negotiable goal that you make that money."

And, she says, you have to commit: "When you really want to change something about your life, you've got to go about it with this hell-bent-for-glory attitude. You just can't kind of, sort of want to make some more money. If you're serious about it, you've got to do the work, get very specific about your goals and get in the emotional state to achieve them."

Don't miss: How No. 1 New York Times best-selling author Jen Sincero went from $28,000 a year to 'making seven figures'

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How this simple mindset shift can set you on your way to getting rich
How this simple mindset shift can set you on your way to getting rich