This paper describes the process that Instaclustr follows to design a Cassandra data model for their customers. While not a prescriptive, formal process it does define phases and steps that their team follows when they design a new data model for their white paper.
Phase 1: Understand the data
Phase 2: Define the entities
Phase 3: Review and tune
As well as defining the process they also provide a worked example based on building a database to store and retrieve log messages from multiple servers.