This is working the back of the sweater? (Ignore the ETA comment that I made. I was misreading.)
As you work across the purl row, you put sts on a holder for the left shoulder, then on a holder for the center neck, and finally work the sts for the right shoulder. Your working yarn is attached to the right shoulder so you continue with that shoulder, do decreases at the neck edge (the edge nearest the center held sts). When you get to the shoulder shaping do the decreases at the armhole edge (the same edge that has the armhole decreases). These decreases should occur at the beginning of the knit rows as RK outlined above. You won't do any decreases on the sts on the middle holder. You'll work with those sts at the Back Neck Ribbing step.