[ivanhoe]

Step by step, part by part, and IMHO it's no different than the onboarding process on any new project. I usually try first to understan a general idea of the whole project, where is what (the structure), a bird's eye view of the business logic and the supporting DB structures. And then with time you dive deeper in areas where work needs to be done. If the code is structured properly usually you can start working on a few related parts without a need to know much about the rest of the system. And tests are there to give you the confidence to refactor and change things freely...