As the U.S. Air Force evolves into an Expeditionary Air Force, its methods for maintaining its aircraft must evolve as well. The authors have developed a simulation tool, the Engine Maintenance System Evaluation (EnMasse), to assess the effect of different policies, such as centralized maintenance, on jet engine intermediate maintenance (JEIM) operations. This report, a user's guide to EnMasse, describes the workings of the various processes (module shop, test cell, etc.) in the simulation model. The model allows the user to track the engine operation and maintenance process from the flightline (in deployed or home locations) through the various maintenance shops and back. The report delineates essential components of EnMasse that the user might employ or modify to build a model for various choices of engine types and maintenance policies.