The quality of software depends on the quality of the performance of the development process. Process documents (or so-called process models) help developers to perform their processes. Formal process models have advantages over informal ones (e.g. project handbooks) including completeness, consistency, and unambiguity. The problem addressed in this paper is the choice of a modeling procedure when formalizing existing, informal process descriptions. A study is presented in which an informal description of an IBM Cleanroom process was transformed into a formal Multi-View Process Modeling Language (MVP-L) model. With phase-, level-, and concept-based modeling three different formalization approaches were evaluated of which concept-based modeling turned out to be advantageous over the others with respect to its complexity and necessary rework.