How do I start a home based business?

I am currently doing payroll and human resources for a company and have been thinking about starting my own payroll consulting business from home. I already know the software I would like to buy and is pretty afordable considering that will be the huge chuck of my business. Just wondering on everyones thoughts as to companies that would be interested in this. I currently live and work in the state of Montana, but would be able to do payroll for multiple states with the software I am looking at. Any opinions or leads would be helpful.