ReqIF is a very powerful file format – almost too powerful: Interoperability between the various ReqIF implementations is problematic, as not all tools support all features of ReqIF. To address this, Prostep iViP, a non-profit, organized an implementor forum to address this. The result is an implementation guide. It provides pragmatic recommendations on how to design a ReqIF-compatible tool. None of the recommendations break the standard. However, honoring the standard will drastically improve interoperability. This paper outlines the topics covered in the Implementation Guide.