The basics steps to follow are:

1) Make a note of all the files. Well named files tell what they do. If they do not, then its probably in the comments. Its important that you know what each file is for.

2) Find the entry point (main, WinMain, _main_)

3) Make a note of all the major macros, structs and functions you see. Note them down. Do it recursively as you go from main towards the other defs.

4) (optional) Read the changelog and the documentation if available.

5) It will be easier to keep track of what you are reading if you note it down. Then go over and read, and keep making notes. Keep detailed but easy to find notes.

6) Please keep making notes.

7) Make notes as you read

8) Did I stress on making notes?

This method helped me a lot :) Happy Hacking