It's superior court...I'm not sure about a course, but I can offer some resources. Q&A on guardianship: http://www.washingtonlawhelp.org/resource/questions-and-answers-on-guardianship?ref=WRNyW
And information on alternatives (I'm sure you know what's best for your daughter, but there are degrees of intellectual disability and guardianship may not be appropriate if there are less restrictive options available): http://www.washingtonlawhelp.org/files/C9D2EA3F-0350-D9AF-ACAE-BF37E9BC9FFA/attachments/392E02E5-DDD9-3257-2013-D64145502759/3302en.pdf
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Here is the website from the Washington courts about becoming a certified guardian. I hope this helps. http://www.courts.wa.gov/programs_orgs/guardian/
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Guardianships are filed in Superior Court. Washington law now requires that petitioners seeking to be appointed as guardians who are not certified professional guardians must view an online training video. You can find information about this on the website for your county's superior court. This video is to be supplied without cost to petitioners.