To install this mod, you will need OpenIV, which can be found here:
https://openiv.com/

Once you've installed it, load OpenIV and point it to your root GTA IV folder if you haven't already. From there, click "Edit Mode" to enable importing files. 

Then, navigate to pc/audio/sfx/resident.rpf and open it. Inside, there should just a subfolder also called WEAPONS which contains a file called "WEAPONS". 

Extract the contents of the zip file and replace the WEAPONS file in resident.rpf with mine by simply clicking and dragging my file into OpenIV. 

And with that, it's installed! Have fun.