We think you’re going to really, really dig the plugin integrations we’re making available later this year :)
HoyleJason Roy commented
I really agree using paper as the first thing. Secondly understand the nature of the business, their process. What value addition the business is bringing to the community or the reasons that they need to maintain DBMS software.
Put their process flow in paper so data architect has fairly have a good picture what is really going-on. Classify/identify Master data and transactional data, with that process you'll know what is key entity and its attributes that's really make them important for the business.
If there are multiple sources or domains, prepare a high level picture and its integration points, and narrow down each domain or subject area. I can add more to it as a step-by-step approach. Here it might be overwhelming.