First, you should have a degree in fashion design and/or business. An MBA from a good business school would help too.
Let's assume you have that- now, you should be an AMAZING artist with really good leather-working skills, and talented at putting fabrics together. Let's assume you're all of that too.
After that, PROMOTION IS EVERYTHING. Make social media accounts for your brand and advertise on social media, pay influencers to promote your brand, etc. It's expensive, but if you want to succeed, you gotta have the money to do it.
Next, offer them at relatively OK prices. Things with quality workmanship that look cool, modern, and trendy, yet are reasonably priced are very popular with the general market. Why do you think Amancio Ortega, the owner of Zara, is one of the richest men in the world? It'll take a lot of time, but eventually your efforts MIGHT pay off...
In the fashion world, there's no guarantee. You might as well work at an already established fashion company and work your way up the ranks. Take Rachel Green from the show "F.R.I.E.N.D.S" as an example :)