Marriageability Affidavit

Hi, I live in Seattle Washington, I will be going to Wuhan China to get married, I need to get a Marriageability Affidavit. I'd like to get in done before I go to Wuhan, my understanding is I can't get it done in Wahun (is this True?) I'd like to get it done with you in San Francisco.

Do I need to make an appointment, what do I need to bring with me.
Regards, Russell