The "advice to new gunsmiths" has been done many times. I will try to summarize:
1. Learn how to run a business; there are community college courses and good books. You don't need an MBA from Harvard but you do need to know that running a business is not the same as tinkering with guns as a hobby.
2. Have enough capital. It is a too common idea that all a gunsmith needs is a mill bastard file, a screwdriver and an FFL. Gunsmith machines and tools are expensive; some estimates of a minimum investment run to $50k and IMHO that is the low end. (A set of headspace gauges for one caliber can run over $100; a finishing reamer close to that.) Even good files are costly.
3. Don't think that you can just farm out the work or rent the tools you need; customers don't want to wait and if they find out you are sending the work out, they will skip the middleman.
4. It may cost, but hire a guy with some gun knowledge to work the front and BS with the customers while you do the real (money) work. If you spend your time debating 9mm vs. .45, you will get nothing done. You will be the friendliest gunsmith who ever went broke.
5. Don't set up shop in your home if there is any other alternative. If you do, you will be right in the middle of a job when the wife needs you to hang a picture or move the furniture. It also will make your home a target for thugs looking for guns, and guys with broken guns at 3 AM on opening day.
6. Unless you have someone you can trust to at least keep open, give up shooting and hunting on business days. You can either go hunting or you can fix the other guy's gun so he can go hunting. Fun or money? If you want to stay in business, the answer is "money".
Jim