There are two ways in which you could be found not guilty of a crime in Washington. The first is a straightforward not guilty verdict from a judge or jury. This is the best case because once the verdict is handed down, you are free and no longer have to worry about the charges against you or any punishment. The other way is not guilty by reason of insanity. This plays out a little different than a straight not guilty plea.
The New York Times explains a not guilty by reason of insanity plea still recognizes some guilt. You do not get to walk away scot-free. Such a verdict, whether by judge or jury, simply recognizes that you were mentally unsound at the time of the crime. It does not mean that you did not do the crime. You just cannot be held accountable for your actions in the way someone found guilty could be.
This plea, though, does not mean you can go back to your normal life. The court is compelled to ensure you get the mental health treatment that you need to prevent any future crimes. You could actually spend more time in a mental health facility than you would if you were found guilty of the crime. This is because an insanity plea means you are confined to an institution until your mental health issues are effectively treated. This is very subjective and could mean you spend decades in an institution. This information is for education and is not legal advice.