Some ideas: Spend time learning how to do thenusers jobs, understanding how thenusers use the system. Go into the code, pick one spot, and understand what it does very well. For that one spot, make a list of all the external things it uses that you don't know about. Go to each spot in that list and make notes about how that thing works, indented under the list item. Repeat that process recursively for a level or two, or as many as you have patience for.

Pair program with someone else who knows the system. Write unit tests.