I'm working a few modular walls and I'm not sure if I'm doing the best workflow for windows, doors and other types of "holes". Right now, I have a standard height for windows, so I created a duplicate of all my walls in all sizes, and cut a hole with the standard window size. The same for doors. So, for every wall segment…
Because this really depends on the style of the architecture it would help to have pictures. For some styles of architecture you can make a small doorframe that you place flat pieces around like you described. Sometimes you've got to make a bigger doorframe and hide the edges.