Ok, first DL a mod. Most mods should be for "mods" folder, these are the ones that can be started with "mod starter". Other mods replace original soldat files, so make a few copies of soldat just in case.
You'll see when you unzip a mod (Like Lazarus project). A interface-gfx folder and an mods folder, copy these 2 directly into ur soldat directory. if u start soldat with normal icon you will have no modified files. But when you use mod starter you'll seethe mod listed, select and click run soldat modded. P.S u need to correctly place file in "Mods folder" things like weapon-gfx weapons.ini... etc must be placed directly into there.
The other mods don't require the use of the mod starter. Landwarrior and ERA for example. They overwrite ur original files.
Hope this helps