Look at similar patterns, like Topsecret from knitty. When you get to the underarms, BO or leave the underarm stitches on a holded, then knit the sleeve, and do the same thing for the underarm stitches. Then knit across the back, take 1 sleeve and knit across its stitches, knit across the front, then the other sleeve. Now they're all joined together on one needle. Decrease the yoke however you want.