Identify the purpose of the model. Who is the model for, why is it needed, how will it be used?
Define the model context. Who are the external actors, what exchanges take place between the process and the external actors, and what are the boundaries?
Outline basic process flows. What are the inputs to, and expected outcomes, of the process?
Define activities in the process. What takes place at each step, who performs that step, what outcomes are reasonable?
Refine and decompose activities as necessary. What additional detail is necessary?
Validate with stakeholders.