Hi:

Starting point ideally is a Context Diagram (a type of Data Flow Diagram) to properly scope out your system.  But, starting there implies a pure top down analysis, and few can do such.  So start with some lower level data flow diagrams and summarize upwards.  Without scoping, scope creep - especially on larger scale efforts - will retard your efforts.

Remember:  Use data flow diagrams to model behavior at higher leves of abstraction, where systems are notorious for being non-sequential.   After you have decomposed downwards far enough, switch over to sequence based modeling techniques (like BPMN)  to then nail down "the system in the small".

Tony

 from my experience, there is no set rule for process improvement. You apply steps based on the project of interested hence tailor you steps according your project most especially the scope. If you do not clearly define your scope you will stray far beyond what you hope / set out to achieve and that of course will have an impact on time lines, cost (depending on your project) and quality.

like  Bernie Hittner stated, its all about improving the As Is process to eliminate waste, increase efficiency, reduce processing time, and keeping your client satisfied.

. ID and Understand the business processes

. ID and understand the associated systems

. ID and understand the relationship associated with the various data sets

. I would recommend you start with a high level process map which will help you scope you boundaries

. then you break it down further  

 Hope this helps.