Install:

Install the folder to "mods/update/x64/dlcpacks/" with OpenIV.
Go to "mods/update/update.rpf/common/data/" and edit DLCLIST.XML file, adding a new line for the new Add-On.

How to use it:
Drag & drop the weapon files into the "weapons.rpf" file located in your Add-On.
Then, you should take a look at how the META files works (I've marked with a comment "<!-- comment -->" where starts and ends
a section) and replace such information with the information of the replace version of the weapon.

As example, if you're going to make an Add-On of a weapon that originally replaces the Assault Rifle vanila weapon, then you just need to
rename the "ak47" words on each file with the name of your new weapon (be careful with the weapon name tag content on META files).
Otherwise, you'll need to look into every vanilla game file and take the specific info of the "replace" version of your weapon and adapt it
to work with your Add-On META files, replacing the content of the examples included on this Add-On.