Mount&Blade Modding is a fairly easy, though time-consuming process, thanks to the core game being designed to be modded. Once the Module Structure and Component Structures are understood, changes can be made fairly quickly. The Mount&Blade series is so easily moddable because the game engine uses all of the Configuration Files and Module Resources to build the world on start-up. This also means that there are some limitations to what can be changed, specifically core game mechanics, control structures, and others.
All mods to Mount&Blade are in the Mount&Blade / Modules directory. The core game is called "Native." Before making any changes, it is highly advisable to backup the "Native" folder, then make one more copy and rename the new one. Components such as menus, items, locations, scripts, and pretty much everything else, uses its location within a configuration file as the key for it to be referenced by anything else in the game. Put more simply, the 13th item in the item_kinds1.txt is a "Tutorial Sword." Every other script, troop, merchant, save-game, etc... will reference (item_kinds:component 13) wherever it should appear in the game. If entries are added or removed from a mod, any existing save game in that mod those references will be different and it will either be buggy, or not work at all. After making any changes to a mod, it is imperative that it is played by starting a new game.
Modding Tools
There are many Modding Tools and methods for Mount&Blade. Compilers such as Taleworlds provides Modding Tools which will generate all of the necessary files into the module. Direct manipulation tools such as Warband Mod Tools which make changes to the configuration files directly. And then there is the ill-advised manually editing the files.