There do not appear to be any specifics regarding what should be included in a petition. It is best to contact your County Elections Department and the County Prosecuting Attorney to determine the exact format they will accept.
A word to the wise: Be sure to confer with your County Prosecuting Attorney before printing any petitions!
Contact information for County Elections Departments in Washington State is available on the Secretary of State's Elections & Voting web page. Contact information for County Prosecuting Attorneys is available at the Washington Association of Prosecuting Attorneys.