Damn, I could've sworn I answered this question already, like, 10 times.
Here's one from earlier, updated for 1.4:
Okay. Get a mod. Open the archive.
If there's a folder inside with the name of the mod, just extract that folder in your Soldat\Mods directory.
If there's a bunch of folders inside named the same as Soldat's folders (Interface-gfx, Sfx, Weapons-gfx, etc.), create a new folder in the Soldat\Mods directory and extract the files into that folder.
You can then use Soldat's built-in Mod Launcher to run the mod. Just open it up and choose the folder which has the mod you want to use. There are l33ter ways to run mods, but you don't need to know them. They revolve around the
-mod parameter.