Once the goals of the project are clear, and when the company knows what they want to accomplish with their brand new ERP, it's time to go into details and write down all of the company processes one by one that will have to be performed or supported by Microsoft Dynamics NAV.
When you think about your processes, don't just expose what they should theoretically be. Ask the people who are actually carrying out those processes about what they really do. Also, ask about the exceptions to the processes, as handling exceptions for a normal process usually requires more time from end users.
You may want to take this opportunity to eliminate exceptions for a normal process by changing how the process works. Exceptions are basically processes that are created to do something that a normal process does not accommodate. So, essentially...