Finger press seam open; fold binding back in half and continue to stitch the open area; stitch overlapping the seams where you started and ended.
looks like the rest of your seams.
Fold to back and hand stitch down. Your corners should look like this.
I hope this tutorial is helpful to you.
corner from front
[ATTACH=CONFIG]111998[/ATTACH]
corner on back of quilt
[ATTACH=CONFIG]111999[/ATTACH]