Burned out from years of building his Denton, Texas-based graphic design business, Simon Trask sold it in 2017. But his payout, while sizable, wasn't enough to support his wife and three young kids for the next several decades. So Trask set his sights on a big but increasingly popular goal: retirement by age 40.

Now 37, he adheres to frugal principles that underpin the red-hot FIRE movement, with its mantra of "financial independence, retire early." Trask and his family drive 15-year-old cars, live in the house they bought at the bottom of the market in 2011, and buy used clothes and toys. But Trask has also found a way to pursue the early-retirement goals of FIRE within his entrepreneurial career, by investing in e-commerce companies that he can run--and spruce up--part time. Trask's first purchase, a website that sells hunting knives, returned 140 percent on his initial investment in the first year; now he's trying to improve the website and product marketing for his second business, Rita Marie's Chicken Coops. "I look for fixer-uppers--businesses I can grow by applying my craft," he says. "To me, it's a much more realistic and achievable goal for financial independence." If you're interested in attaining the same sort of early retirement lifestyle, here's how to get started.

1. Create your nest egg.

Zach Hendrix, 37, of Nashville, put aside 10 percent of his landscaping business's revenue on the first of every month--before he paid any bills or invested in the business. "The habit forced me to save and invest anywhere from $10,000 to $50,000 per year," he says. Hendrix used those savings to buy 13 rental properties, which today provide enough cash flow for him to live comfortably and, he says, "pursue the projects I want to pursue." Owning investment properties is a common strategy for those seeking early retirement. Toronto residents Leif Kristjansen and his wife, Lina, spent five years saving about 55 percent of their annual income as a lab scientist and a librarian, respectively, in order to put $500,000 toward seven rental properties. Today, in their mid 30s and retired for two years, they spend $5,000 a month on living expenses and bring in $3,000 in rental income. They also have other investments, and to bring in another $3,000 monthly, Lina runs a site that sells baby products, while Leif provides retirement advice on a FIRE blog.

2. Daydream in detail.

"If you're going from 100 mph to 0 at a very young age, consider how you're going to spend your time," says Michelle Maton, a certified financial planner and partner at the Planning Center in Chicago. Kristjansen, for example, says his expenses have actually gone up in retirement, because he and his wife have more time. That's not uncommon for young retirees who are still healthy and active, says Michelle Brownstein, a certified financial planner in San Francisco with Personal Capital, a digital financial adviser. "Travel is often a big-ticket item," she says. So get specific about your retirement dreams, and start putting them in motion--whether that means setting up part-time work or buying your first rental property--before you take the full plunge.

3. Embrace scenario planning.

Maton recommends that those looking to retire early ensure they have enough cash to cover two years of living expenses; income for the following three years in a combination of short- and intermediate-term bonds; and the rest of their portfolio spread across a mix of diversified assets, including stocks. That won't prevent all long-term hiccups. "Look at health insurance," says Maton. "The markets are changing, there are legal changes, and it's very hard to model what the costs will look like over an extended period of time." Brownstein encourages her clients to run various scenarios through Personal Capital to come up with a monthly budget in their comfort zone. And she warns that it's best to be conservative with expenses when your savings must last for decades. "I advise my clients to err on the side of overestimating their costs," Brownstein says. "Maintaining a cushion is always important."