Measure the height of the window you are building the shutter for. Measure from the top inside casing and exterior sill. Measure the width of the window from the inside of the casing. Divide that amount in half. This is the finished height and width of each shutter (based on a single double-hung-type window).
Cut 6-inch boards to the height you need. You may need three or more boards depending on the width of your window. Cut two batten boards out of 4-inch board. These should be the width of your finished shutter.
Position your 6-inch boards so that the tops align. Space them no more than 1/2 inch apart to reach your optimum shutter width. Since your shutter is decorative and not functional, you can make it narrower to keep the spaces between your boards no more than 1/2 inch apart. Simply recut the battens to your new width.
Position the battens across (perpendicular to) your shutter boards approximately 1/4 the distance from the top and the bottom. They should be evenly spaced from the top and the bottom. Screw each batten to the shutter boards with two offset deck screws per board.