To make mods, you need LFS Editor, which can be downloaded from the Downloads page. The 3D model itself can be modelled in another editor of your choice (e.g. Blender) which can then export an OBJ file and imported into LFS Editor. Or you can find a 3D model made by another author on the internet, but it has to have the correct license to be used in LFS and can't come from another game without permission.

Here is the manual page for LFS Editor, which contains more subsections about the editor, video tutorials, guides, FAQ etc.:

https://en.lfsmanual.net/wiki/LFS_Editor