Understitching can create a more polished look for necklines, and can help prevent a neck facing from shifting out of place.
Finger press the seam allowance up towards the neck facing. This is in preparation to stitch the neck facing to the seam allowance.
Make sure the seam allowance is pressed towards neck facing.
Start at shoulder seam, understitch 1/8'' from edge of neckline seam.
End understitch about 1'' from center front.
Note under stitch is not on outside of shirt.
Do the same technique to the other side. This side is a bit trickier, you will need to invert neckline point in order to place sewing machine foot in correct position.