Install a flexible arch window curtain rod in the window casement of the arch window you are making the fan curtain for. Follow the manufacturer’s direction for installing the rod.
Measure straight across the bottom of the arch of the window. Add 1 ¼ inches for seam allowance. This is the length measurement. Then, measure the perimeter of the arch. Double this measurement and add 1 ¼ inches for seam allowance to get the width measurement. Cut your fabric using these length and width measurements.
Iron the edges of your fabric rectangle toward the wrong side ¼ inch all the way around. Stitch this hem in place. Iron the edges another 1 inch toward the wrong side. Stitch this fold in place as close to the previously hemmed edge as possible to create the pocket for the curtain rod.
Insert the flexible curtain rod into the hem across the width of the curtain panel. Lay the curtain panel flat, with the curtain rod across the top. Place a chalk mark on each side of the panel half the distance from the bottom of the arch window to each top corner.
Fold the sides behind the curtain panel. Pin the sides together at the chalk marks. Doing so will bend the curtain rod to the shape of your arched window.
Gather the excess fabric together in the middle over the pin. Tie the fabric into a sort of ponytail, and secure with string. Adjust the gathers of the curtain panel evenly along the curved curtain rod.
Twist the two ends of excess fabric around each other. Turn this twisted fabric around itself until it forms a small, tight bun, like a hair chignon. Tuck the end under the bun, and pin in place. Stitch the sides of the curtain panel at the pin point on the back. Hand stitch the bun in place. Now your fan curtain is ready to hang in your arched window.