Use some of the suggestions below:
The power of communication is often underestimated, try to talk to the author(s) of the code first.
Then, read through design documents provided by the authors. However stop if the documents seem to make things more confusing than help you understand.
Make use of note and diagramming tools, such as the SQL Server diagram tool, to make understanding the code more intuitive.
Learning the framework or development style used (MVC, N-tier, REST api) can be very useful to understand how code work if it follows certain programming style.
Do a debug and step through the code, see how things connect with each other. Don’t abuse this method ’cause it can be time consuming.
Hack around to get the job done.