I didn't use a pattern for mine, but you will most likely have to draft something for the sleeves anyway. The skirt is relatively straightforward, it's just the sleeves that might give you issues. I would look into Renaissance-type costume patterns; that might help.
Metrocon '11: Robin & Amon (WHR) [Sat Only]
AFO '11: Undecided
Spooky Empire '11: Jack & Sally [Sat Only]
Working on: Jack & Sally, Jasmine
Planned: Poison Ivy, Bellatrix Lestrange, Ayame, Briar Rose, Azula, Scarlet Witch, The Witch (L4D), Teela