I would think that you would go about it the way you would start any other business.

1. Define the objectives and purpose of the origanization
2. Define the geographic area and clientele
3. Determine the material and staffing needs of your organization
4. Secure financing either from foundations, government or other sources
5. Like any business prepare to have headaches along the way, lots of 'em

I think the mistake people make with Non Profits, is the assumption they do not have to run them like businesses, but you need much of the same entreprenuerial qualities to make it succeed and sustain itself.

Best of luck!