Well, as a minimum, a text file and a banner. There may be other images (sprites) too in a mod.

There has been some requests for Steam workshop (also in the main forum), but, for now at least, this modding sub-forum is the place to look for mods.

Each mod is just a ZIP file that you download and then unpack in the directory that opens when you select "Open Mods Directory" at the bottom of the Mods screen (after you select Mods in the main menu).
(On Mac and Linux, this directory is ~/.coe4/mods.)

This should result in a c4m file (plain text file) directly in your mods directory, plus a subdirectory containing the banner and possibly other stuff.

Then (re)start the game, select the Mods menu, and click on the mod to enable it. (Right-clicking on it should give you some info, unless the mod author forgot to include a description.)

Originally posted by steven.aus:

What tag do you need to use to give a description?

The command is