One of the most simple operations to perform in 3D modelling is also one of those that seem to cause the most people consternation. As a result, here are various methods to create windows and portals in objects.
As I use more methods in my daily work, I may add them to this tutorial if I feel that they are significantly different and not just variations on a theme.
The bulk of this tutorial has been taken from one of the unpublished Blender for the Faint Hearted tutorials that I've not had time to finish. The tutorial is however, mostly about principles, so while this is a Blender tutorial it should have some benefits for other 3D application users are well.