First, make sure you can build and run this code. Open Source is usually good about this. Next, pick a path and start tracing through the code. Let's say there'

First, make sure you can build and run this code. Open Source is usually good about this. Next, pick a path and start tracing through the code. Let's say there's a GUI at the front and DB at the back. Find a simple form and trace the "save" button all the way back to the DB. Finally, just start making some small changes.