I just passed the test last month.
Go here: http://www.nacesplus.org/washington.html
Fill out the application and mail it to:
NACES/Washington State NNAAP
7600 Burnet Road, Suite 440
Austin, TX 78757-1299
There is also a list of regional testing sites on the website that I have listed above. Be sure to send in your completed app, a copy of your CNA course certificate, and the fee of $107 (for both the written and skills exam). Also, be sure you send a certified check or money order. I made the mistake of sending a personal check and that delayed my scheduling.
Lastly, make sure you apply to be on the Washington OBRA-NA registry. I think this can be found at www.doh.wa.gov/Nrs_asst
When you pass, NACES will send your results to the people at the registry. Since they have your name on the registry already, then the process of receiving your certification should be expedited.