When two triangles are adjacent in the Delaunay triangulation, draw a line segment between the circumcenters of these triangles.
There is an easy to remember rule for the duality between Delaunay triangulations and Voronoi diagrams: The dimensions of the dual elements always sum up to the dimension of space. For example in 2D:
Delaunay <-> Voronoi
Triangle (2) + Point (0) = 2
Segment (1) + Segment (1) = 2
Point (0) + Cell (2) = 2
...similar in 3D.