understanding a program project

[Tracubik]

Hi all,
i hope not to be too much OT with this request.
I'ld like to modify/contribute some open source in python, but first i've 
to read and understand the code.
So, is there some guide lines / procedure to follow to help me in this 
process.
I remember at school time there was some schema or something to create to 
display the interaction of different functions / modules

My idea was to create a model with all the methods and arrows to link 
they...

any link of post explaining some "learning procedure" or "programs 
schema" would be really appreciated

thanks
MedeoTL