I have preached this sermon before, but here goes again. IF you decide to get into business, get into business. Do it right. You will need more than an FFL. You will probably need local licenses and permits as well as comply with zoning laws, etc. Find out what they are and if you are ready to make a substantial investment. And, as noted above, don't forget liability insurance. Also have a policy to deal with the guy who wants you to fix a dangerous gun (say a Damascus barrel shotgun), or to put on a too light trigger pull, or ream out a cheap revolver to .357.
I would recommend not using your home as a place of business unless you like being awakened at 4 AM on the morning of opening day by some guy who just now remembered that he broke a firing pin last November.
One of the first things you should do is find a gun-friendly lawyer and put him/her on retainer. Lots of wheels turn more easily for lawyers than for plain old citizens. ("Mr. Police Chief, Section 22, Para 33 of the state code says you have no power to deny a license....)
Then, if you can't do it yourself, find someone to do the books. You may think of gunsmithing as a hobby, but unless you have plenty of money and intend to run the business in the red for fun, you need to keep good books and charge properly for the work you do.
Cost of equipment is astronomical unless you can find good used equipment or buy a going shop. Just a set of chambering reamers can run over $150. Yet going cheap will mean poor work and disappointed customers. So you will need capital in big gobs if you do things right. You can start small, accepting little jobs, but you don't want to become known as a "little job" guy or no one will bring you the bigger (and more lucrative) jobs.
One of the biggest traps of a person going into any business from a hobby is the tendency to "do favors" for friends. Believe me, those "friends" will eat you alive if you let them. Example: Joe sees a gun advertised at $200 dealer cost and asks you, with your FFL, to take delivery. He paid $200 and expects to pay you $200, while you eat the shipping costs, the tax, and your time and trouble.
Whether you sell goods or labor, charge everyone the same price. There is no way to build ill will like Joe finding out you charged him more for scope mounting than you charged Pete for the same job. Joe won't ask you if there was any difference in the work, he just won't come back and will badmouth you all over town.
You may want to do retail sales as well as gunsmithing to keep a good cash flow. But do not man the counter yourself. Hire a kid who knows something about guns or a retired guy to work the front. Otherwise, you will get nothing done except listen to stories about 1000 yard shots at squirrels.
You might notice that I didn't yet mention actual knowledge. Others who have taken various courses will be better able than I to give you advice in that area. Books and videos can help, but hands on experience is best. The best way may be to find a local gunsmith and work part time for little pay while you learn. Most gunsmiths started out learning on the customers' guns, but that can be a very expensive way to learn if you goof and have to buy the guy a new Baer pistol or (heaven forbid) a British double rifle.
Jim
