Start with a LOT of money to buy liability insurance. Take a trade school course in gunsmithing, buy a bunch of expensive tools, rent a business location, get an FFL license, a business license, and tax ID number.
The most basic answer is, open a gun store and advertise. However, it is more common for a person to go to a gunsmithing school, get a certificate, then go to work for a gun store to get experience, then eventually start their own business.
Gunsmithing is when a person can repair, rebuild and restore a gun. To become a gunsmith, you will need to take classes. Penn Foster offers a course in gunsmithing.
I found a really good site that holds the information about really good Gunsmithing schools in Michigan. http://www.technical-schools-guide.com/michigan-gunsmithing-schools.html
"Yes, there are some gunsmithing schools near Boston. One of them is the Institute of Tech in boston. This school has many classes such as gunsmithing. This school also has classes on other subjects."
H. E. MacFarland has written: 'Gunsmithing Simplified' -- subject(s): Gunsmithing
Making and repairing firearms.
I have researched this topic for you and found that there are indeed online gunsmithing schools. Some that you may want to consider are Penn Foster Career School and Ashworth College which both offer online gunsmithing courses.
You would start as an apprentice to a gunsmith, and learn the craft of gunsmithing while helping the gunsmith to make or repair guns.
Metalurgy
Ralph T. Walker has written: 'Black powder gunsmithing' -- subject(s): Gunsmithing, Muzzle-loading firearms
It is not mentioned in the question as to which area is local to you. But in U.S. the Trinidad State in Colorado offers good gunsmithing courses. National Rifle Association also offers good short term gunsmithing courses. Also you can take online courses from http://www.americangunsmith.com/
Depends on the type and application.