When you are sewing a 2 piece collar together (collar and facing), you will get better results if you sew the parts together in 2 steps.  First sew the long seam.  Trim it to 1/4" - 3/8", then press the seam open before pressing it flat so the the facing is not visible from the right side.  Then turn right sides together again and sew the collar ends.  Trim and clip the corners.  You might need to also trim excess away from the neck edge of the facing.