Obtain this from the court where you were convicted. It should be mailed to you when you complete probation/community supervision. But you may have to go the court where you were convicted. Even though your voting rights have been restored, the Certificate does not restore your gun rights in the State of Washington.
Obtain a copy of your judgment & sentence from the court at the same time, if you do not have it.
Obtain your criminal history from the WSP at their website called WATCH. It costs about $10.00 so have your credit card ready. Your lawyer will appreciate that you did your homework. It helps to have your NICS history, too. You can obtain this at the FBI's NICS site. It is more important to get this ahead of time if you have convictions from jurisdictions outside Washington State.
Many counties in Washington have step by step guides if you want to do it yourself. We will be glad to meet with you and answer any questions in order to get you started.