Measure the top edge of the mirror. Divide this measurement by two and add 1/2 inch.
Lay the table runner on flat surface and mark the center of one-long edge with a pin. Measure the distance you calculated along the edge on each side of the pin. Mark both these points with a pin.
Mark the center of the other side with a pin. Subtract 3 inches from the measurement used on the other side. Measure the new distance on each side of the pin on this edge of the runner. Mark these points with pins.
Draw a line with tailor's chalk from the pins on one side to the pins on the corresponding side. Use a yardstick to ensure the lines are straight.
Set a sewing machine to sew a straight stitch with a long-stitch length. Sew along the chalk lines.
Pull up the bobbin threads to gather the runner until the chalk lines are one-third the original width of the runner. Adjust the gathers evenly.
Set the machine to sew a standard stitch length and sew along the gathered seam to hold the gathers in place.
Center the drape over the top,of the mirror with the side with the longest distance between the gathers at the top. The drape should hang down for 1 inch behind the mirror.