Just do exactly what it says, one step at a time, you're going to just work on the sts in the middle of the piece for the back of the head, then sew the edges of that 'flap' to the edges where you bound off the sts.
Bind off 30 sts, then finish the whole row. Turn and BO 30 sts and then you're left with the 39 sts in the middle. Just work on these sts with the decs at the edges, until it's the same length as the sts measure along the cast off edge - not their sides from the beginiing of the piece. So if the 30 sts measures 6", then you work until the back also measures 6".