Knowledge base for modding standard firmware

Hi again guys,

where can I find the required knowledge base to customize the standard firmware?

I read the only article present on this site belonging this and I downloaded the code and toolchain too, but I need to study something before to approach the mod.

I'm not a newbie of the I.T. world because I'm an active software developer but without any kind of experience about modding firmwares for routers.

This would be a training to improve my knowledge, so any resource is really appreciated: forums, ebooks, articles, required prerequisites and so on..

Moreover, I'd like to know if a virtual environment exists to test the new firmware before flashing it..