Take a wooden board and fix a white sheet of paper on it with a cellotape .Somewhere near the center of the paper,draw a straight line AB with a the long face of the plane mirror along the line AB and hold it in vertical position with plasticine.Fix two common pins P and Q in front of the mirror in a upright position.The distance between the pins should be 4 to 5cm .Looking from the side B of the plane mirror,fix two more pins R and S  such that these pins and the images of pins P and Q are in the same straight line .Now remove the mirror.remove the pins P,Q,R and S one by one and draw small small circles around the pinholes .Join PQ and RS and produce them to meet at a point O which is the point of incidence. At O,draw ON perpendicular to the mirror surface.Since the incident ray PO ,the reflected ray OS and the normal ON lie in the plane of the paper all the three are also on the same plane.