Ever increasing complexity is one of the key challenges for software-intensive systems. All phases of the system engineering lifecycle are affected; thus, appropriate methods, techniques, and tools must be provided to manage this increasing complexity. Another challenge arises from the fact that requirements are traditionally documented in natural language. This is especially true for legal artifacts such as laws or standards, since organizations are required to consider these kinds of legal requirements when bringing products to the market. Applying model-based engineering approaches is one possibility to address these problems, but methodical uncertainties caused by a lack of method support have so far prevented their widespread use in early project phases. This paper addresses these uncertainties by introducing a systematic approach for modeling legal artifacts such as laws or standards. The method is assessed in the context of emission legislation in the automotive domain