Alright, here's the easiest way to do it.
First, you need to put the BRF into the Resource folder of your module, Native in this example, and the DDS files into the Textures folder. Make sure you're putting them in the module and not the main folder, it should be Mount&Blade/Modules/Native (or whatever mod)/Resources.
Second, you want to make sure the Module is reading the BRF. Open up the module.ini, it's basically a text file, and you should see a lot of lines that say load_resource = 'stuff'. Go to the last entry like that, I think it's barrier primitives and under that type in " load_mod_resource = Narf's_Plate_Armour ".
Thirdly, if you want to just replace an in-game item with Narf's model like I did, open Item_kinds.txt and find the entry for plate mail. Control + F is useful here, you'll also want to make sure you don't have word wrap on.
Find the bit that says full_plate_armor and replace it with milanese_plate.
Native's plate armor will now use Narf's Milanese plate model.
You can do this with anything else, but you'll need to find out the names of the models inside the BRF, which you'll want to download OpenBRF to figure out, just reverse engineer these steps to do it with other models and items.