I worked on the right side of the jacket.
This is what a finished sleeve looks like. I put one of these gathers on the front and one on the back of each sleeve. (Four gathered sections in total, 8 seams).
After wearing this a few times, I realized that the slightest tugging on the sleeves would break the stitching. To fix this, I sewed a seam directly over the gathers, halfway between the two gathering seams. So far, it seems to hold.