To start modding look at the Mod Tools folder under the game files.
There's a Tiled folder that contains the map editor. It can be used to edit the .tmx room designs in the Rooms folder.
There's an example mod folder that contains a VS project. Install free Visual Studio Community Edition to open the .sln solution file. Be sure that Game Development with Unity is checked when you install Visual Studio.
The VS solution will create a .dll file that should be copied into a /Mods folder under the game executable folder. The mods are scanned when the game starts.
Currently mods are restricted to other games with mods. Also, mods require Fellowship to be purchased.