How do you write a letter to stop harassment in Washington state?

To write a letter to stop harassment in Washington state, clearly identify yourself and the individual you are addressing. State the specific behaviors that you find harassing, include dates and incidents if possible, and express that you want the behavior to stop immediately. Mention any relevant laws if applicable, and state that you may take further action if the harassment continues. Keep the tone professional and assertive, and consider sending the letter via a method that provides proof of delivery.