Once you've the circle right side out, pressed it, and sewn up the opening, bring it back to the sewing machine.  Sew a line 1/4-inch from the edge of the circle all the way around. Then sew a second line 1/4-inch to the left of the first line, 1/2-inch from the edge of the circle.

Using sharp scissors, cut a small slit between one line and the other, making sure to cut only the top fabric.  Cut another slit directly across the circle from the first.  These are the holes the cord will pass through.