Measure the distance across one side of the archway, just below the arch, with a tape measure.
Purchase a curtain rod that is equal to the distance measured inside the archway. For example, if the archway is equal to 5 feet, purchase a 5-foot-long curtain rod.
Screw one curtain rod bracket on each side of the inside of the archway, just below the arch.
Measure the distance from the brackets to the floor or the base of the window. Purchase a curtain that is equal to this distance. For instance, if the distance from the brackets to the floor is 7 feet, purchase curtains that are 7 feet long.
Insert the curtain rod through the tabs of the curtain. Place the curtain rod on the brackets.
Straighten the curtains. Pull them closed when you want privacy, and pull them open when you want to see the view or allow passage through the doorway.