The Cal Letter requires an application and a hefty fee (nearly $600 last time I checked). The pertinent information can be had at:
http://www.medbd.ca.gov
or
Medical Board of California
Ronald Joseph, Executive Director
1426 Howe Ave., Suite 54
Sacramento, CA 95825-3236
(916) 263-2389 / Fax: (916) 263-2387
(800) 633-2322
Follow the links under Information for Applicants and then International Applicants.
Information for other states can be obtained at:
http://www.fsmb.org