Khalil, there is a document known as " NGOSS Real World Use Case" that may be helpful (GB921R Release 8.0)
In our experience, we start with the operations processes decomposing the levels 3 into possible levels 4 using the descriptions given in the GB921D. Then, we analyze the “as is” processes, and the key point here is to identify the boundaries that the model defines and understand its logic.
Let me know if you need something else.
(Sorry for my english)
BR
LEandro