What is the best way to open a payroll company for small businesses?

What's the best way to open a payroll company for small businesses?

The best way to open any business is, obviously, with a big order from a big client on day one.
Assuming most people will never have this, write a plan, raise the funds, hire the people, and go to work. In your case, of course, your software will be paramount to managing growth and success.
Make sure you not only have a good system for running the business, but also a great system for marketing and selling new customers. Marketing and sales are generally the most overlooked areas for new business owners.