The angle of relection must equal the angle of incidence.
Measure the angle of incidence from the normal to the incident ray.
Mark the same size angle from the point of incidence on the other side of the normal.
Draw the reflected ray to form the proper angle of reflection.
Label each angle with its measured value (e.g. 17.5 degrees)

Now you have successfully shown the reflection of one ray of light with a concave mirror.
Repeat steps #2-4 on the same diagram, but now draw the incident ray 2 cm below the optical axis. (parallel to OA)