Place a plane mirror on the table in a standing position. Take a piece of paper and make a small hole in its centre. Make sure that the light in the room is not so bright. Hold the sheet normal/ perpendicular to the table. Take another large piece of paper and place it on the table horizontally in contact with the vertical mirror. Draw a normal line on the large piece of paper from the mirror.
Now, light a torch on the mirror through the small hole such that the ray of light falls on the normal/ perpendicular line that touches the bottom of the mirror. When the ray from this hole is incident on the mirror, it gets reflected in a certain direction.
Now we can easily observe the incident ray, reflected ray and the normal to the mirror at the point of incidence on the large piece of paper placed on the table. This shows that the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.