In recent times, research has moved towards using VLF radio transmissions propagating in the earth-ionosphere waveguide as a detector of a variety of transient geophysical phenomena. A correct interpretation of such results depends critically on understanding the propagation characteristics of the path being monitored. The observed effects will vary depending on time of day, path length, path orientation, magnetic latitude and VLF frequency. This paper provides a brief tutorial of the relevant propagation dependencies for medium to long VLF paths best understood in terms of waveguide mode theory together with results either not previously published, not published in the open scientific literature or whose significance has been little recognised.