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'Shark Tank's' Kevin O'Leary: Do this simple exercise to be financially successful in 2018

Ali Montag

If you hope to see more money in your bank account in 2018 than you did last year, you'll need a strategy to get there. While tackling your earning, saving and spending habits can seem daunting, it doesn't have to be complicated.

In fact, it can be as simple as doing this exercise — all you need is a piece of paper and a pen, "Shark Tank" investor Kevin O'Leary tells CNBC Make It.

"Take a 90-day period in your life, three months," O'Leary says. Add up all of the money you earned or received. "Gifts you get, the job you've got, if you work a part-time job, if you sell stuff on eBay, whatever it is, list every source of income over that three-month period."

Then, on the other side of the sheet of paper, tally your expenses.

"Make sure you capture every dime you spent in that period, I don't care what it is," he says. " A double latte , I don't care — list it."

Taking a look at October, November and December of 2017 will help you get a sense of your consumption habits overall, O'Leary explains.

"A three-month period is a really good snapshot of what your lifestyle is, including all the purchases you make in clothing, in fashion, the places you go, the vacations you take. Every use of funds," he says.

The final step is to compare what you earned with what you spent. If you've been spending less than you made, you're on the right track.

"However, if you are actually spending more than you brought in [during] that 90-day period, you're screwed," O'Leary explains. "Because you know where it's going? It's going on to your credit card, and you're paying 20 to 25 percent [interest]." (On average, credit cards charge about 16.7 percent interest, according to Bankrate.com.)

Fellow sharks Mark Cuban and Robert Herjavec both agree that credit card debt is a pitfall to avoid. Cuban advises not using credit cards altogether, while Herjavec suggests paying them off every month.

"When I was young, I would carry a balance on my credit card," Herjavec tells CNBC Make It. "My advice — pay off your credit cards every month and therefore pay no interest. Credit card interest is probably the most expensive loan you could ever get."

If the last three months of 2017 reveal you've been overspending, O'Leary advises making a change now to start 2018 on the right foot.

"The biggest mistake in life people make is their lifestyle — their lavish lifestyle is more than they are actually making," he says. O'Leary suggests checking in with your spending to avoid that mistake. Then, set a goal for yourself to take it a step further.

"The best thing would be actually to only spend 90 percent of what you bring in, and invest the other 10 percent for the future ," he says. "That, will save you in the long run."

Don't miss: Why 'Shark Tank' investor Kevin O'Leary refuses to spend $2.50 on a cup of coffee

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Disclaimer: CNBC owns the exclusive off-network cable rights to "Shark Tank."