Google search a mod, or 'Warband mods' and of coarse multiple.
First off open up your browser(I recommend chrome) In all seriousness, download the mod, extract it until it looks like a legitimate module folder, and plop that into C:\Program. Start Mount and Blade: Warband and check the Modules drop-down box for the mod you installed. Copy extracted files to Warband's Modules folder.
