Application of process-based models beyond the research community has been limited, in part because they do not operate in an intuitive graphical user-friendly environment. This article describes the procedure of adapting a spatially explicit biological-process model, MAESTRA (Multi-Array Evaporation Stand Tree Radiation A), to run in a standard graphical user interface (GUI). The methods used to adapt the MAESTRA model are generally applicable to other individual tree process-based models and, therefore, should simplify other coupling attempts. The three primary changes to MAESTRA were the placement of the MAESTRA code inside a Microsoft Windows API (application programming interface) function called WINMAIN, rearrangement of the input file structure to fit the hierarchical file structure used by the Graphical User Interface for Crop Simulations (GUICS), and the addition of iteration counters to read array-based data read for each array within the MAESTRA input files. The procedure of adapting MAESTRA's input structure and interface design should allow other forest stand process-based models to work within GUICS.