Measure the inside of the window casing.
Cut 1-by-1-inch board to the length of your measurement. This board is called the batten or header board.
Insert the batten against the underside of the top window casing as close to the window frame as possible. Screw 3-inch-long wood screws through the batten and into the window casing. Screw every 8 inches.
Measure the width of the window and add 2 inches.
Cut a 1-by-1-inch board to your measurement using a table saw.
Position the batten so it's centered on the face of the window trim 1 inch above and 1 inch beyond the inside edge of the window trim. Screw the batten to the window trim using 3-inch wood screws through the face of the batten and into the face of the window trim. Keep your batten level with the trim.