How do I proceed to authenticate documents required for China?

First, have your documents issued in Japan (but not by a Japanese government authority) such as Affidavit, Declaration, Power of Attorney, etc. notarized by a local Notary Public and then to be authenticated by the Japan Academy of Law. If your documents are issued by a Japanese government authority (e.g. Marriage Certificate, Birth Certificate, Confirmation of No Criminal Record), then have it authenticated directly by the Japan Ministry of Foreign Affairs.

Next, take the documents authenticated by the Japan Academy of Law or Japan Ministry of Foreign Affairs to the Chinese Visa Application Service Center in Japan for authentication.