Measure the height and width of the door frame. Subtract a quarter inch from the measurements to allow the door to swing freely.
Transfer the height measurement to the 1-by-12 boards. Use a yardstick to mark a straight line across the top of the boards at the right height. Cut off the excess wood with a saw.
Lay the two 1-by-12 boards on a flat surface, side by side. Measure the width of the boards, then transfer the measurement to two 1-by-6 boards. Cut the 1-by-6 boards to fit.
Lay the 1-by-6 boards on top of, and perpendicular to, the two 1-by-12 boards, 1 foot from the top and bottom. Screw down through the 1-by-6 and into the larger boards with deck screws.
Lay the third 1-by-6 board diagonally across the other 1-by-6 boards. Align the outer edge of the diagonal board with the corners of the horizontal boards, making a Z shape.
Mark the line where the boards intersect the yardstick. Remove the diagonal board and cut it to the angle. Replace between the horizontal boards and screw down with deck screws.
Add hinges and a latch to the door. While a helper holds the door in place, screw the hinges to the door frame.