Sometimes these patterns sacrifice simplicity in order to make the pattern more concise. Result is confusion. You can work these shoulders at the same time with 2 balls of yarn or go with the pattern recommendation to work one shoulder after the other.
Cast off 4sts at the beginning of the row. Then work toward the center, casting off the center 9sts and continue to the end of the row. Work this shoulder first.
Turn at the end of the row, cast off 4sts then finish the row, turn and cast off 10sts (neck edge).
Work to the end of the row (armhole edge), turn and cast off 4sts.
Work to the neck edge again, turn and work back to the armhole, turn and cast off the final 4sts.
Now join a new end of yarn to the other shoulder at the neck edge and bind off 10sts then finish the row.
At the armhole edge, bind off 4sts, work to the end of row, turn work back to the armhole edge.
Now turn and bind off 4sts.
The bind offs of 10 sts will be at the neck edges and the bind offs of 4sts will always start at the armhole edge. I make a diagram of the sweater to show the bind offs just so I can keep them straight and then check them off as I work them. It’s a good reminder of which are at the neck and which at the shoulder.
That sweater is too adorable!