Start with creating a business entity. Either sole proprietorship or LLC is really all that makes sense for a small business like selling BPs.
Once you have registered your business entity, you will need to register a fictitious business name with the county you reside in.
After that, if you live within the city limits, you’ll need a city business license. When you travel to shows you’re supposed to purchase a license for any city you operate in.
Next you need to apply for a resale permit so that you can properly collect and pay the sales tax associated with each sale.
Once you have a business entity, fictitious business name (DBA), and city business license (if applicable) you can get a business checking account which most point of sale services require before they will handle credit card transactions for you.
There are some other details but they will present themselves along the way. I have a video explaining the process on my IGTV