approach for designing a generic,extensible and modular C++ application [on hold]

I have to create a C++ application which will convert some text files or xml files to pdf reports i.e. it will take some xml/text file/files and converts the data in those files to some tables and put it in pdf file which will be our output.

This application will have some configuration stored in a xml file which has some configurations values which provides flexibility to configure the output.

I can create an application but I don't know whether it will be modular or extensible from design perspective.

I would like to understand from designing perspective how to approach this programming problem, to write a modular and generic code and what all things to keep in mind before actually started coding.

I am not asking for a solution here but I would like to get some pointers for creating a good application design which should be generic, modular and extensible.

Are there any rules which we should keep in mind ?