Are you making a button placket at the left shoulder? I would need to see the pattern to be sure, but it sounds like you would start picking up at the front left (as worn) edge where the shoulders would be connected if you were sewing them together and pick up working left, across the front, then on around past the right shoulder seam and then on to the back and around to where the other shoulder edge is in the back left. You probably have stitches for the back neck on a holder, and maybe some in the front as well, when you come to them you knit them off and then continue picking up stitches on the other side of the holder.