Screw the hinge to the side of the upper-left corner of the white board. When you buy the white board, make sure it has a border of wood, metal, or plastic (wood works best) at least a 1/2 inch thick so you can screw the hinge to the border.
Paint the piece of lumber with black and white stripes. The stripes help the editor see the moment of impact on video. You can paint corresponding stripes on the top border of the white board, too.
Screw the lumber onto the top half of the hinge. You should be able to lift the lumber lever freely above the white board and produce a solid clapping sound when you strike the white board with the lever.
Draw a line with the dry-erase pen down the middle of the white board. Write the word "Scene" at the top-center of the left side. Write the word "Take" at the top-center of the right side. At the bottom of the white board, you can also write in other information about the film, such as the working title, the director's name, and perhaps the date of the shoot.
Begin each shot by announcing the Scene and Take numbers for the sake of the audio, then clap the clapboard to indicate the beginning of the shot.