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  It's time to start looking at all the flows of data/documents
  into, out of, and inside your system.  This includes
  interactions between classes.
  
  You should be able to see that every input either comes from
  outside the system or from another class.  Every input should
  be the responsibility of a class.
  
  Similarly, every output or service should be generated by a
  class and either be the input of another class or leave the
  system.
  
  In both cases, check/identify the essential behaviours of the
  classes involved, their responsibilities, and any
  collaborators.
  
  Any unused classes are either redundant or indicate missing
  inputs, outputs, or interactions.  Genuinely redundant classes
  should be deleted.
  
  If you have two or more people, you might like to act out your
  model.  You'll be self-conscious at first, but this is worth
  overcoming.  Try this with a user group if you can, as
  everyone should benefit.
  
  Assign classes to each other, and see what happens with each
  input.  If your model works, you'll be able to track the
  input's effect properly.
  
  To check that outputs get produced properly you have to
  identify what triggers them.  Things like monthly reports are
  easy: it's a particular time/date, or a user tells you to
  produce the reports.  Some outputs are so hard to trigger that
  you should already be asking yourself if your model is right,
  and how you're going to test your final implementation.
  
  With plenty of people, you can pick one whose job is to ask
  you questions about how things get processed/produced.  Pick
  someone who's good at asking awkward questions (your best
  tester, probably).
  
  If this feels like a waste of time, remember that mistakes found
  early on typically save vast amounts of implementation time.