A pair of cream pants gave Sara Blakely an idea that made her a billionaire.

When Sara Blakely was 27, she was selling fax machines door-to-door, and cutting the feet out of her panty-hose to wear beneath her clothes.

One made her money, one would soon make her creation a household name with a few more dollar signs to boot.

Of course, in cutting out the feet of her panty-hose, Blakely was making the first unofficial prototype for the hosiery we now know as Spanx.

Spanx would make Blakely the youngest female billionaire in America.

But before Spanx became how we know it today, Blakely was a little-known sales rep from Florida, trying to sell a particularly female product to boardrooms full of men.

In an interview with the podcast How I Built This, Blakely explained how a pair of cream pants began a long road to becoming a self-made entrepreneur and, eventually, a billionaire.

“I had spent a lot of money on these cream pants –  about US$98 which for me was a lot of money – and they just hung in my closet unworn, because every time I would go to wear them you could see the under garment,” she told host Guy Raz.

Regular underwear didn’t work, nor did a g-string and shapewear was too thick and “overdone”. So she saw an opportunity for something in the middle, and cut out her own make-shift shapewear from regular panty-hose. The light bulb, she said, went off.

“I had set aside $5000 that I had saved selling fax machines door-to-door and that’s what I started Spanx with,” she said.

She had never sewn, had never taken a business class in her life and certainly didn’t know anyone who worked in the fashion and retail industry. But there was a hole in the market, and she was going to convince everyone worth knowing that she intended to close it.

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“The manufacturing plants didn’t get the idea at all. Then it occurred to me that the only people I had been meeting with were men and it became clear why they had been so uncomfortable for so long. The people who were making the garments weren’t spending all day in them. So I had a hard time explaining why this product was so important [because] they couldn’t wrap their minds around it,” she told Raz.

Meeting after meeting, her pitches fell on deaf ears. Then she got a phone call. A manufacturer by the name of Sam wanted to take a chance on her – he had consulted his three daughters who each saw the brilliance in her idea.

Interestingly, Blakely didn’t tell her friends about her dream. She didn’t want to hear the criticism while the idea was so “vulnerable” in its “infancy”, and so, in that first year, Blakely embarked on a very solo journey to growing her business.

What is obvious is that Blakely made some very deliberate decisions along the way that make Spanx how stands today.

To save some cash, she bought a book called Patents and Trademarks and wrote the product’s patent from scratch. The name of the company itself came to her while in the car after almost two years brainstorming the perfect one. She wanted it to be memorable. She wanted it to have a ‘K’ sound – like the two most widely recognised companies in the world Coca Cola and Kodak did – and she wanted it to be trademarked easily.

And she didn't want Spanx in just any stores - she wanted only the best. The first place she called? One of the biggest retailers in the country, of course. Neiman Marcus was hit with an onslaught of phone calls from an ambitious hosiery designer now located in Georgia. And after a while, they listened. If she could get on a plane to Dallas, they would give her 10 minutes of their time.

"In the middle of the meeting I could tell I was losing her but I figured this was my one shot. So I said, 'You know what Dianne, will you come with me to the bathroom?' She said, 'Excuse me?' I said, 'I know, I know it's a little weird but will you just please come with me I want to show you my own product before and after.'

She wore the cream pants, and showed the buyer how the product worked.

"Wow, I get it. It's brilliant," Blakely recalls her saying. Spanx was to be put in seven stores around the country.

Within two years, the company had made about $10 million and Blakely was still living in her old apartment, unfazed by the sheer volume of cash coming in and her accountant's repeated pleas for her to buy a house and to find somewhere else to live.

To this day, Spanx has never done a formal ad campaign and most remarkably, 15 years on, Blakely has never taken the company public. She owns 100 per cent of Spanx.

"It's very weird. It still feels very surreal to me."

You can listen to How I Built This by NPR here.