The purpose of building a model of a is to simplify and mimic a real-life situation so that appropriate action may be taken.
starts with an idea. Something is noticed or a change becomes necessary to an existing system. So the requirements for the model are decided on. What must be achieved?
From these requirements expected outputs from the model would produced. A requirement would be that management would be informed and the output might be a report, as an example from computer systems, although they may be the same thing, such as a ticket might need to be generated.
The requirements and outputs lead naturally the inputs and processing which would be needed to generate the outputs.
The applicable constraints regarding cost or operating capacity or any other limitations and the legal, physical or other environment need to be determined. These play a big factor in any model.
It is best to start with as simple a model as possible and test it with examples to determine feasibility and then progressively refine it. The more it is tested with different examples the better the end result will be. Iterate through the developement, refining and testing at each stage. These can be simple examples and do not need to be complex tests, but also help to provide a basis for testing the final model and system.
Avoid at looking at any current system if possible to avoid become fixated on what is currently being done. A fresh look at the problem is required and the solution developed can later be compared with that existing to determine any discrepancies. if something is being done which is not in the model, determine why it is being done. it may no longer be necessary, like the human appendix - once useful but no longer needed.
Lastly, in developing any model or system a bias to action is needed as well as an open mind. Do not be afraid of making mistakes. Fear of failure is an impediment to developing a good model. The path to the perfect model is paved with detected and corrected errors.