Scooted to Rome 2 forum...
The mods often come as .pack files. These pack files go in the game directory: Total War Rome II\data
You can access the game directory if you don't know it by entering steam, going to the game's properties, Installed Files, and hit Browse...
The mod should then appear in the Mod Manager when you launch.
Some mods are more intricate, for example the "AUG MOD" must be extracted from the archive (should be very easy: enter the archive, pull the folder into the data directory or a temporary one you make for this purpose. If windows does not behave you can use a tool like 7zip to do the same). The contents within the aug mod folder go into the game directory, and you can use the batch files to select what changes to use. It comes with a neat uninstaller to bin the mod in one go if you don't like it.
Anything further would depend on the mod. Note that unfortunately, older mods may simply not work because they were made for an older game version and may never be updated. I'm no expert at troubleshooting Rome 2 at all but this is the general gist.