Summarize the following:
You must marry within 60 days of issuance. After the license expires, you must start the application process all over again.  You can get married in any county in Washington. You are not limited to the county from which you received the marriage license. You must be married by a proper officiant, which is prescribed by state law. These include active and retired supreme court justices, judges of the appellate and superior courts, superior court commissioners, any regularly licensed or ordained minister or any priest of any church or religious denomination, and judges of courts of limited jurisdiction. The officiant should sign the license. You also need two witnesses to sign. They should both be over age 18. Within 30 days of the marriage you must return the signed license to the county office which issued it to you. You can purchase certified copies from the Auditor's office.

Summary:
Marry soon. Get the license signed after the marriage. Return the signed license.