Buy one (or more) boat or contract with boat owners for use of instruction boats.
Step2
Secure an insurance policy that specifies liability for rental and instruction. A recreational policy won't cover you.
Step3
Develop curricula for the course(s) you intend to offer. A national certifying organization such as the American Sailing Association can make this step easier by providing you with their standard course materials.
Step4
Join a certifying organization. It isn't necessary to offer nationally recognized credentials but there are some tremendous advantages.
Step5
Select course materials including, text books, log books and exams. A certifying organization will provide these items.
Step6
Prepare promotional materials such as brochures and advertisements.
Step7
Contact state and local authorities to determine whether they require licenses and permits.
Step8
Open for business.
Tips & Warnings
Don't rely on revenue for, at least, a couple years. Start with enough money to survive two years without making a penny. If things go well you'll be pleasantly surprised. If not, you'll still be in business.
If you think owning a sailing school will provide you with an easy lifestyle, think again. The only way to make it work is to treat it like serious business.