Measure the length of the curved rod and use enough fabric width to gather nicely on the rod. You would first sew the bottom hem and then lay the material out flat on a table. Transfer as many as five height measurements from the arch onto the fabric and sew from point to point creating a curved double hem at the top. Once it is gathered on a rod the valance would conform very nicely with the arch. By placing curved hems on the top of a pair of panels the same effect can be achieve. They can then be allowed to hand straight or tied back.