I’m sure that a lot of developers find it harder to read existing code and understand the ideals behind it, rather than writing it from scratch, themselves. I too have this same problem, and so started looking around online as to how people have dealt with it. I’ve compiled some good answers from various sources, below:

• Try and understand at a high level, what the purpose of the code is, and then identify the key elements/functions that help achieve this purpose
• Try to read as much design and documentation about the project before diving in
• Try and figure out the relations between various components and then draw/visualise an architecture diagram
• Talk to authors and clarify things with them beforehand
• Go through unit tests to get a high level overview
• Start with the entry point in the program. This will in turn point to what gets started and sometimes how things are linked.
• Make small edits or bug fixes, then gradually build up
• Keep notes. Don’t jump in and start developing; it’s more valuable to spend time understanding than to generate messy or inappropriate code.

Hope you find success after following the above tips :)