To overcome a shortage of flexible and low-cost solutions for wire arc additive manufacturing (WAAM) preprocessing, this work´s objective was to develop and validate an in-house computational programme in an open-source environment for WAAM preprocessing planning. Algorithms for reading STL (stereolithography) files and implementing rotation, slicing, trajectory planning, and machine code generation were elaborated and implemented in the Scilab environment (free and open-source). A graphical interface was developed to facilitate user interaction, with 5 options for path planning. The functionality of each work step is detailed. For validation of the software, single and multiple-layer prints, with different geometrical complexity and printing challenges, were built in a CNC table geared by the generated machine code. The validation criteria were deposition imperfection, morphological, and dimensional tolerances. The outputs showed that the parts were successfully printed. Therefore, this work demonstrates that Scilab provides the necessary resources for companies and universities to implement and/or develop algorithms for planning and generating trajectories for WAAM. Moreover, emerging ideas can be reasonably easily implemented in such software, not always possible in commercial packages.