Register for a workshop (face-to-face/online) through IB in the subject area you are interested in. Unfortunately, the only face-to-face PD scheduled to happen in Japan in the near future is the March PYP PD at Tamagawa. I wouldn't expect much else to appear as many of the schools trying to get authorization are already past the staff PD stage and MEXT has stopped subsidizing them. However, there are upcoming events all throughout IB Asia Pacific - e.g. Hanoi, Beijing, Mumbai, Adelaide, Kuala Lumpur, Melbourne, and Hong Kong. You can also register for the online workshops, but those generally have supplemental work that needs to be completed and the pacing can be tad slow. (I have yet to meet anyone who prefers those over the face-to-face ones).

Like /u/carocaracalla said below, you need to make sure you are committed to foster IB Learners and support IB-style inquiry-based learning. Getting certification is one of the paperwork requirements for better employment, but as the MYP/DP Coordinator, part of my work responsibilities are making sure candidate teachers really understand the IB and are able to produce tangible results when they teach the students (especially in regard to the DP).