[run2arun]

Find out how to run unit tests in your project and then find interesting unit tests to run. Set break points preferably at some launch points of your code and debug through unit tests. The process will take you through relevant parts of the code, the decisions being made etc.

This method has also worked for me when building features in a large codebase. Write a unit test first and then keep checking where the code breaks and fix those until your test succeeds. This is effectively TDD. Note that you might have to refactor the code for better design but it gets you started towards understanding the flow.