There are a couple of ways to do it.
1: Start at the outside border, quilt to the inside border, stitch in the ditch to your next start point, stitch back to the outside.
2: Start at the outside border, quilt to the point you want to end, stay on the same line and quilt back to the outside border - a double stitch. This is the easiest way I have found to do piano keys when the inside in on a curve, such as a swag in your border.
And...most importantly, don't forget to use a ruler!!