Hmmm, I thought a Ruana was like a wrap, and held in place w/a broach?
I did also find this, called a ruana jacket is that the sleeves you want? If so, you can follow the first pattern (or this one!) and simply seam the front to the back leaving arm holes, as described in finishing for this one.
Edit - if there is a specific pattern you want to modify, post a link here and perhaps someone can help you modify the size to fit you!