You can hand the notice to the tenant.
You can attempt to locate the tenant, if not finding him, post and mail.
RCW 59.12.040 is the law that tells you what you have to do:
ny notice provided for in this chapter shall be served either (1) by delivering a copy personally to the person entitled thereto; or (2) if he or she be absent from the premises unlawfully held, by leaving there a copy, with some person of suitable age and discretion, and sending a copy through the mail addressed to the person entitled thereto at his or her place of residence; or (3) if the person to be notified be a tenant, or an unlawful holder of premises, and his or her place of residence is not known, or if a person of suitable age and discretion there cannot be found then by affixing a copy of the notice in a conspicuous place on the premises unlawfully held, and also delivering a copy to a person there residing, if such a person can be found, and also sending a copy through the mail addressed to the tenant, or unlawful occupant, at the place where the premises unlawfully held are situated. Service upon a subtenant may be made in the same manner: PROVIDED, That in cases where the tenant or unlawful occupant, shall be conducting a hotel, inn, lodging house, boarding house, or shall be renting rooms while still retaining control of the premises as a whole, that the guests, lodgers, boarders, or persons renting such rooms shall not be considered as subtenants within the meaning of this chapter, but all such persons may be served by affixing a copy of the notice to be served in two conspicuous places upon the premises unlawfully held; and such persons shall not be necessary parties defendant in an action to recover possession of said premises. Service of any notice provided for in this chapter may be had upon a corporation by delivering a copy thereof to any officer, agent, or person having charge of the business of such corporation, at the premises unlawfully held, and in case no such officer, agent, or person can be found upon such premises, then service may be had by affixing a copy of such notice in a conspicuous place upon said premises and by sending a copy through the mail addressed to such corporation at the place where said premises are situated. Proof of any service under this section may be made by the affidavit of the person making the same in like manner and with like effect as the proof of service of summons in civil actions. When a copy of notice is sent through the mail, as provided in this section, service shall be deemed complete when such copy is deposited in the United States mail in the county in which the property is situated properly addressed with postage prepaid: PROVIDED, HOWEVER, That when service is made by mail one additional day shall be allowed before the commencement of an action based upon such notice. RCW 59.18.375 may also apply to notice given under this chapter.
I disagree that certified mail is a proper form of service of this notice unless the lease specifically calls for service by certified mail. The statue says that if you cannot find the tenant, you may post the notice in a conspicuous location and send a copy to the tenants via regular first class mail. But isn't certified mail better? No. Sometimes certified mail needs to be signed for and they will not deliver the notice until someone is available to sign for it. While certified mail does prove when you sent the notice, the statute says you must send it normal US mail. Because this is a time and manner requirement (how you serve a notice), strict compliance with the statute as the legislature wrote it is required in order for the notice to be effective.
As long as you make sure to send it via normal US mail, you may go above and beyond all you want. You could do both regular and Certified mail, regular mail and telegram, regular mail and notice delivered by horseback, etc . . . so long as you at least send it regular US mail.
Hope that helps,
This answer is for informational purposes only and should not be construed to establish an attorney client relationship. Before taking any legal action, it is always advisable to discuss your specific situation with an attorney.