Draw the principal axis as a straight horizontal line.
Draw the lens or mirror at the center of the axis.
Mark the optical center for a lens or the pole for a mirror.
Mark the focus and center of curvature if needed.
Draw the object as an upright arrow on the principal axis.
Draw one ray parallel to the principal axis.
Reflect or refract that ray according to the mirror or lens rule.
Draw a second ray through the focus or toward the focus.
Reflect or refract that ray according to the mirror or lens rule.
Draw a third ray through the optical center or center of curvature if needed.
Extend the reflected or refracted rays until they meet.
Mark the image at the point where the rays intersect.
Use a dotted line to show virtual rays if the image is virtual.
Label the object, image, focus, and other key points.