Shown below are concentric solid and dashed circles, each different in radius by 1 cm. Consider the circular pattern a top view of water waves, where the solid circles are crests and the dashed circles are troughs.

On a sheet of paper, draw two sets of concentric circles with a compass. Choose any part of the paper for your center (except the present central point). Let the circles run off the edge of the paper. (optional diagram)
 Find where a dashed line crosses a solid line and draw a large dot at the intersection. Do this for ALL places where a solid and dashed line intersect.

With a wide felt marker, connect the dots with smooth lines. These nodal lines lie in regions where the waves have cancelled  where the crest of one wave overlaps the trough of another.
