Millennial who banked $1 million in 5 years: Don't make these 4 mistakes when trying to earn extra cash

Grant Sabatier has mastered the art of the side hustle. By age 30, he had earned over $1 million, in part by working up to 13 side gigs at a time.

The Millennial Money founder and author of "Financial Freedom: A Proven Path to All the Money You Will Ever Need" did a wide range of things to earn extra cash: He flipped domain names, bought and sold Volkswagen campers, invested in a moving company, and more.

If you want to start a successful side hustle, or several, Sabatier says, there are four mistakes you should avoid.

A successful side hustle can bring in a substantial amount of money. But it's important to prove that your idea is profitable first, Sabatier tells CNBC Make It. That means starting small, whether it's building up a client list before starting a dog walking network, or selling an online course before expanding your web business.

"Always do that before you invest a lot of money in launching your side hustle," he says. That way, "you know because you've sold a few that there are people out there that are willing to buy from you."

Sabatier warns against sinking too much money in right away. If you decide to take on a side hustle without prior experience, dropping $10,000 on equipment or spending thousands on a custom-built website could put you into debt.

It's crucial to start with a project that suits your current skill set, especially if you're new to the side hustle world, Sabatier says.

"While everyone means well, a lot of people have really ambitious side hustles," he explains. "They're like, 'I have this incredible app idea and I'm going to launch it and it's going to be incredible. But I don't know how to code, and I've never built an app, and I don't know how to do it.'"

Jumping straight into a complicated idea can leave you feeling overwhelmed and unmotivated. When people start with a difficult project, they often end up wasting months building something they aren't able to complete, Sabatier says.

"They should have picked something a little bit simpler that they could have gotten started doing," he says.

In most cases, you're not going to build a successful company overnight. It's important to understand the reality of how long it will take to begin generating profits, Sabatier says.

"People want to make money quickly. That's why they start side hustling," he says. "But in reality, any type of side hustle that you do is going to take some time to build. Maybe, depending upon how fast you work, three to four weeks or a couple months. You're probably not going to make money right away."

And while it might be easy to envision yourself spending every spare moment working on your side hustle, how much time are you actually able to commit? Before you say yes to a side gig, it's also vital that you're realistic about how many hours a week you're able to invest, Sabatier says.

If you want to earn extra cash, choose something you'll enjoy spending your free time doing. "You should always start by trying to make some money doing something you love first," Sabatier says.

He explains: "A lot of people look online and they see a side hustle that someone's doing, and they're really successful and they're like, 'I want to do that. I can make money doing that.' But in reality, if you don't enjoy what you're doing, you're not going to stick with it, and you're not going to have much fun."

Basically, Sabatier argues, if you just copy someone else's idea, it's not necessarily going to be successful for you, no matter how well it works for them. You have to do what's going to engage, and work for, you.

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