This party perfect bolero is started out worked in one piece, then separated for the fronts and back.
On page 13 you start with the complete body, then divide for the armholes having 23(25-27-30) sts for one front, 46(52-54-60) sts for the back, 23(25-27-30) sts for the second front.
When you work the back on page 14, you go back to the 46(52-54-60) sts that were left in the center of the work. Join ball of yarn to the first st with wrong side facing and work the back of the sweater as the pattern reads.
Hope this helps, happy knitting.