In order to take an examination, you must first obtain a BSDCG ID. The same ID can then be used to register for any BSD certification exam. There are no pre-requisites to take an examination--in other words, you do not have to be BSDA certified in order to take the BSDP examination. Read the "Intended Audience" section for each exam to determine which exam(s) match your skill set.
You can help contribute to the development and maintenance of certification exams by purchasing the BSDA Study DVD which contains the latest stable releases of the 4 BSDs, the BSDA and BSDP exam objectives, as well as the Handbooks, Guides, and FAQs from the BSD projects.
Additional (free) resources can be found on the BSD Wiki.