I don't have any particular advice about what is required to be a freelance
lighting designer, but I have looked pretty extensively into being a freelance software developer and I'm about 90% sure that tax wise and legally they are the same. How to attract clients and run a business? That's the different part that I can't help with.
The term "freelancer" probably means "self-employed" and your legal requirements are pretty simple, as far as I can tell. I am not a lawyer and you should consider finding one to help you if you are going to make your entire living this way. You definitely don't want to end up in trouble because you miss-filed paper work or followed an internet strangers advice.
As a self-employed person you still have to pay income taxes AND self-employment tax AND follow any state regulations and taxes on self employment. The IRS website is actually pretty helpful in figuring out what that means.
https://www.irs.gov/businesses/small-businesses-self-employed/self-employed-individuals-tax-center
I could sum up what I think that
page says, but since you definitely don't want to follow my legal / tax advice you should probably just read it yourself.
Then you will need to file paperwork in your state of employment. I live in MT so I will describe what I would be required to do if I wanted to start my own business and pay my own taxes. Because there is no professional liscensure requirement as a software developer (like there is for lawyers, electricians, contractors, gunsmiths,
etc...) it's actually as easy as following the federal requirements here.
When I was actually looking at doing that I did talk to a real business person / tax professional and they were going to help me do some other things like file articles of organisation to form an LLC (which means that I would pay my taxes slightly differently and not be personally liable for damages caused by software I wrote). In MT it costs $140 to file articles of organisation for an LLC. I don't know what it would cost in your state and like I've said:
You should really consult a tax professional or CPA who is familiar with the laws in your area, you definitely don't want to screw this up. It could cost you a lot of money if you don't pay taxes correctly or are found at fault for damages caused by your work.
You should talk to a tax professional or CPA or lawyer or some otherwise businessy educated person who is familiar with the laws in your area.
The good news is that there are probably lots of small business owners in your area and almost all cities and towns have organisations set up to help people who want to start a business. There are lots of resources available that will offer your more specific and legally sound advice about how to become self employed. There is even a branch of the government who's job it is to help you do that. It's called the small business administration:
https://www.sba.gov/
Good luck, you should definitely talk to a real business professional. I don't think I could repeat that enough. But it's not scary, once I started looking into it for my own business I realized it's actually pretty simple.