Designing natural ventilation systems involves different levels of sophistication in calculations and software toolsaccording to the system complexity and the stage of the design process. This paper provides a general workflow to model these systems, while describing the types of resources available for different complexities and design stages. Inputs needed at each step of the workflow are discussed, and, when not available anywhere else, ways to compute them are also presented. Particularly, this paper shows how to account for the ventilation efficiency of various window types and pressure losses of obstructions along the airflow path when their discharge coefficient is unknown.