When I cosplayed Raine from Tales of Symphonia, I took measurments of every single part of my body. Bust, waist, hips, neck, chest, shoulder width, from the center of my back to my shoulders, from my shoulder to my elbow (on each side) and from my elbow to my wrist, from neck to waist, and waist to knees and anything else I could think of. Then I just sketched out the parts of the jacket using my own measurements to get it the right length etc onto pieces of newspaper I taped together. It's cheaper than buying tissue paper, and sturdier, too. The only tricky part was the sleeves, but since my dad sews, he helped me out with those. So as long as you know basically how the jacket goes together and what each piece looks like, and you're a decent artist (I'm not, but my costume was all about the straight lines), then it should be a challenge, but not one you won't be able to handle.

Then again, there's always the mix and match with pre-made patterns. :D

Good luck!