7 steps to get rich, from a personal-finance classic the millionaire founder of Nasty Gal calls 'one of the best'

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Before Nasty Gal founder Sophia Amoruso reached millionaire status — she's worth an estimated $250 million — she was broke, dumpster-diving, and a frequent shoplifter.

She credits part of her financial revival to George S. Clason's "The Richest Man in Babylon," she explains in her book, "#GIRLBOSS."

The 1926 personal-finance classic offers money advice in a collection of parables based in the ancient city of Babylon. It follows the story of Arkad, the son of a humble merchant who grows to become the richest man in Babylon. Arkad then shares the "secret to wealth" with the rest of the city, starting with what he calls the "seven cures for a lean purse."

Here are the "seven cures," in Clason's words and ours.

'Start thy purse to fattening': Save 10% of your income

Getting rich all begins with paying yourself first. More specifically, set aside a minimum of 10% of your earnings, Arkad advises: "For every ten coins thou placest within thy purse, take out for use but nine. Thy purse will start to fatten at once and its increasing weight will feel good in thy hand and bring satisfaction to they soul."

Anyone — rich or poor — can put money aside and let it accumulate, Arkad assures his class. You just have to commit to setting aside a minimum of 10%, and you'll learn to live without it.

Today, it's even easier to learn to live without a certain chunk of your income, thanks to technology. You can automatically deposit money from your paycheck and checking account into a retirement account, savings account, or other investment vehicle, removing the temptation to spend. If you never see it, you'll learn to live without it.

"I, too, carried a lean purse and cursed it because there was naught within to satisfy my desires," the richest man in Babylon explains to his class. "But when I began to take out from my purse but nine parts of ten I put in, it began to fatten. So will thine."

'Control thy expenditures': Spend less than you make

The next step is to simply spend less than you earn, which is easier said than done. Our consumer-driven society makes it incredibly easy to overspend — and what's more, when income increases, people have a tendency to boost their spending. It's called "lifestyle inflation."

"What each of us calls our 'necessary expenses' will always grow to equal our incomes unless we protest to the contrary," Arkad explains. "Confuse not the necessary expenses with thy desires."

To control your expenses, you have to become a conscious spender and recognize where your money is going. A good starting point is to record all of your purchases (whether in a notebook, through an app like Mint, or on an Excel spreadsheet) and analyze your spending patterns.

"Study thoughtfully thy accustomed habits of living," Arkad says. "Let thy motto be one hundred percent of appreciated value demanded for each coin spent." Even if you're well on your way to accumulating a fortune, the habit of living below your means still applies. There are a surprising number of frugal billionaires who choose to save or give to charity, rather than drop their money on jets, yachts, and mansions.

'Make thy gold multiply': invest

Once you've made a habit out of controlling your expenses and setting aside at least 10% of your income, put that money to work.

"The gold we may retain from our earnings is but the start," says Arkad. "The earnings it will make shall build our fortunes ... Learn to make your treasure work for you. Make it your slave. Make its children and its children's children work for you."

Making your money your "work for you" is the modern-day equivalent to smart investing. You can invest through your employer's 401(k) plan or other retirement accounts, such as a Roth IRA or traditional IRA. Thanks to compound interest, your savings can grow tremendously over time — the trick is to set aside money regularly and to start as early as possible.

If you still have money left over after maxing out retirement accounts, you can research low-cost index funds, which Warren Buffett recommends, look into the online investment platforms known as "robo-advisers," and consider investing in the stock market.