Winston cones have traditionally been used as detector feeds in far-infrared cosmological experiments, such as SuZle, the Sunyaev-Zei'dovich Infra-red Experiment  on the CSO. They are usually designed using ray tracing, which becomes a very poor approximation when the number of spatial modes propagated by the horn is small in number, often the case at the longest wavelengths. We describe a more accurate approach involving electromagnetic modelling of Winston cones using a rigorous electromagnetic mode matching technique. It is straightforward to also consider the case of few-moded corrugated Winston cones, which offer lower sidelobe levels than smooth walled cones which is important for high sensitivity experiments. Furthermore, the mode matching technique allows more complex structures such as back-to-back Winston cones and the detector cavities to also be analysed.