Mark a 24-inch line lengthwise down the middle of the door at one end to indicate where to install the center rail. To the left of the line, mark ¼-inch-thick perpendicular lines at 7, 11, 17, and 24 inches from the end of the door. Mark another ¼-inch-thick line 18 inches from the end on the right side of the centerline. Apply painter's tape on either side of each line, and mask the surrounding areas with newspaper. Use a plastic putty knife to seal the edges and prevent paint from bleeding underneath. Then, with a can of black satin spray paint, coat the lines with light, even strokes.