Fastest ways of becoming familiar with plenty of source code?

Question

I wonder if you could suggest me some effective ways on how to quickly become familiar with big amount of source code.
In the case of code maintenance of existing source code which have poor and/or incomplete documentation.
Some of the ways are code reading, step by step debugging, painting UML class and sequence diagrams or something else...
What else? Are there more efficient?

Answer 1

go through the header files and while reading code note down whatever you understood and underline portion which you could not understand , clarify them with original coder/project lead

Answer 2

the best way is to print the source code and dream that u have read the whole thing and understood.