Pin down the back of the head, starting with tucking whichever side ended up on the bottom of the two folds already sewn down. You may need to fold a dart in to get it to lay flat.
Fold under the raw edges of the fabric of the other side and pin down (blue pearl and red matte pins).
Fold under any other raw edges still visible and pin down any excess (blue glass and red pearl pins).
Try to make sure you completely cover the scrap fabric inside. If you end up without enough fabric, you may not have made the running stitch around the neck far enough down, but it's ok; try to get the nape of the neck covered and you can always cover the back of the head with hair or a sewn on hat, just sew down the edges around the gap.