Background Patient-specific instrumentation (PSI) improves accuracy of surgical operations. PSI needs software for preoperative planning and instrument design. In this study, we explain the methodology of developing a software tool for PSI guide design and preoperative planning in reverse shoulder arthroplasty (RSA). Methods Approaches used to prepare input data, transform them into meaningful features and use of those features to create special guide geometries are explained by describing different algorithms and libraries. Results The developed software is tested on three different patients' data. Preoperative planning is performed and guides designed by software and the patients' bones are manufactured and tested for RSA. The method of building a software is presented to do the preoperative planning and designing specific guides for each patient are shown to be properly functional. Conclusions This study proves processes in the development of the PSI software and the design of a specific guide for RSA.