The Internet is an evolving force that contributes to rapid economic expansion worldwide. However, each newly emerging Internet technology requires rigorous evaluation and testing; a process that often includes simulation and modeling. OPNET Modeler is among the foremost software products for the simulation and modeling of communication protocols and Internet technologies. However, OPNET's considerable amounts of source code and supporting application programming interfaces (API) can be quite overwhelming, even for experienced developers. This paper attempts to demystify the process of modeling in OPNET and enumerate the key steps for creating new simulation models using the OPNET Modeler software package. In this paper, the authors share experiences from their foray into developing a new IP layer mechanism for QoS support. Additionally, this paper details a methodology for expanding the OPNET Modeler network layer implementation.