It's not as hard as it sounds, but it does take some planning. You'll first need to decide if you want to do it from home, or from a commercial/industrial location. From that decision determine the distance to schools and parks (you can only be so close) and contact the city and county to determine any licenses or permits you'll need and contact the state to register your name. Home adds a few other considerations such as HOA restrictions. If you decide to work from home make sure you specify that you're a home occupation, not a business that needs signs and customer parking. Also if you rent you'll need to secure your landlord's permission for the business as the ATF will contact them to make sure you have their permission.
Apply for your FFL. Don't waste money on an FFL kit, everything you need can be found on the ATF.gov website. Buy a gun safe. It can be something as simple as a Stack-On cheapy at first as it just needs to satisfy secure storage. Plan on an ATF visit prior to your license approval. Don't be afraid, they're there to help you get off on the right foot.
Go to Vista Print and get some real business cards. Make a website, no business is legit these days without one it seems.
I would wait if possible before buying tools and setting up shop for power, lighting and ventilation until you have your license in hand. That way you can be sure you'll be putting those things to use for a living.
I've been a home gunsmith for 9 years and have even moved the shop 3 times so I'm pretty good at the little details and steps you need to take. If you need some help shoot me an email at firstname.lastname@example.org
and I'll help you.