Right the RS row begins with a dec at the armhole edge, then you would dec on the WS row at the same edge, then another dec on the next RS row. That's for the Left shoulder (as you wear it), you also do the neck decreases at the same time. If you're also doing the Right shoulder you dec at the end of the RS row, the beg of the WS and end of the next RS row at the armhole edge. While doing the neck decreases for that shoulder too.
sue- knitting heretic