After you've made sure that you'll be in compliance with state and local laws and regulations.
Some states treat black powder guns as firearms, others don't, and if you're setting up a business, there will be general legal hoops you need to jump through, as well. You'll also need to have liability insurance.
It's a good idea to consult a lawyer when setting up any business; I'd say it's essential for someone who wants to manufacture anything that goes bang.