Your question is very general, and there is no general answer, as there are various mods, that you can mod differently.
Basicaly you go to res_mods/0.9.4 (or whatever your version is) and search for config files, .xc, .json, sometimes .txt, or .ini... and edit them with notepad++.
Better ask more precise question, about certain mod or thing you would like to achieve, and learn it that way.
Also download the XVM, unzip, go to doc folder, and read all english files.