Starting a chapter is quite simple. Find people on our lists or elsewhere who live in your area and might be interested in joining.
Once you have some people who are interested, you should arrange semi-regular meetings. Holding official meetings once per month is usually standard, with unofficial meetings happening more spontaneously as people decide they want or need them.
Email and give them contact details for your chapter (your email at a minimum, or a chapter website if there is one) so that they can link to you from the chapters webpage.
There are more suggestions for starting a chapter at here.