Remember to keep working in the back loops only.
The rest of the costume's upper front ain't nothin' you haven't seen before:
Row 18A: ch 1 and turn, sc in the second ch from the hook, 4 sc, 3 hdc (8)
Row 19A: ch 2 and turn, hdc into 3rd ch from the hook, 2 hdc, 5 sc (8)
Rows 20A-23A: Repeat rows 18A-19A twice.
Your piece should look like this after the last row: