This is the story in tutorial form of the creation of a program, Gwx, for building economic models, both macroeconomic and multisectoral. The program closely resembles the G7 program. Like G7, it is written in C++ but it is built with free, open-source, cross-platform tools. There are many books about how to use some specific computer program but few about what was going on in the mind of the designer and programmer as the project evolved, as problems emerged and were solve. This is such a book. The program, Gwx, is designed to be used with the author's book, The Craft of Economic Modeling.