Knowledge about system life cycle processes constitutes one of the most valuable assets for systems engineering practitioners. While effective management of processes is imperative, it is an onerous task as knowledge is tacit and implicit. Although best practice to ensure the quality of such processes has been documented in the International Standard ISO/IEC/IEEE 15288, it is limited to human-readable descriptions and not a computer interpretable knowledge representation. This leads to a problem with interoperability as process descriptions are interpreted in different ways. Consequently, we must define these processes in an explicit, unambiguous way. The goal of our research is to improve the current situation by advancing the semantic formalism of the standard from natural language description to computer readable knowledge presentation - an ontology. This paper proposes a methodology to realize this approach. The methodology ensures the concepts in the system life cycle processes are defined in an unambiguous, systematic and structured way. Through this methodology, an ontology of these concepts is generated, which provides a classification that enriches the perspectives by using different groupings for different viewpoints and purposes. The study contributes to the creation of a semantic network for systems life cycle processes to allow a more systematic and comprehensive understanding of the relations hidden in the processes. It also supports tailoring the system life cycle processes to different systems engineering projects to ensure the best practice.