Treat it like a Normal window:
For a wall of arched windows that you would like to cover with window treatments, hang a straight rod above the windows the length of the entire wall. Then hang curtain panels that cover the entirety of the windows and wall when the treatment is close. When open, it will reveal as much window as possible. Ideally, when the curtains are open, only the wall in between the windows should be covered with drapes.
For a single arch, hang a rod above it the window that is wider then the arched window. This way when the curtains are opened, you can reveal the beauty of the entire window.
Adapt regular curtains:
Regular pocket rod window treatments panels can be hung on a curved rod. You can find curved curtain rods or rods that you can shape in your local hardware store or by mail order. Once the curtain is hung, you will then need to cut the bottom straight across and re-hem it. You will want to purchase extra long panels for this.
Hang tab top curtain panels from antique hooks installed around the arch of the window. Cut the bottom straight across and re-hem.
Custom shutters:
You can purchase wooden shutters with that includes a sunburst top that fills the arch attractively or custom shutters that curve at the top to fit the arch.
Install shutters or blinds across the window leaving the arch exposed. Then hang a decorative window scarf in the arched section. This is done by installing three hooks or drapery ornaments. Install one on each side, and the third one at the top center or off to one side depending on your personal preferences.
Chances are you picked a home with arched windows because you thought they were beautiful. Unless privacy is an issue, why not leave them bare or at least nearly bare. You can add interest or coordinate them with your décor by hanging a simple swag treatment above them, or you can place a standing translucent floor screens in front of them to filter light.
To give the windows some extra interest when you do not want to cover the window, you can add panels of fabric between or on either side of the windows hung from attractive hooks or window ornaments.
Create a curved window where you do not have one:
If you do not have an arched window, but want to give the impression that you do, you can create a straight top, arched window treatment by taking a regular window topper or cornice board and cutting an arched shape out of the bottom edge.
Generous curtains that co-ordinate with an upholstered screen add a touch of flamboyance to a dressing room, while a fluffy rug softens a wooden floor for extra comfort