This paper provides novel fundamental tools for the open-source design and documentation of benchmark network models. The procedure for model development is motivated and described in detail in all its different phases: after an initial discussion on the relevance of open-source benchmark models, particularly in the context of an interoperable power system, the general conceptual framework for model documentation, based on the Holistic Test Description and PreCISE paradigms, is presented. Building up on this theoretical basis, the procedure for model development is structured according to a testing-oriented approach, providing clear examples on how to map and translate the proposed conceptual framework into a practical and complete model description. Finally, to demonstrate the applicability and usefulness of the proposed method, a complete example of model documentation for a developed low-voltage network is described and validated in simulation. The complete model description is publicly available online, as guide and reference for the proposed model documentation procedure.