After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
Drop that main folder of the mod to assets/mod_overrides. The assets folder is supposed to be in your game's installation folder (don't know where that is? click here). Create the folder if necessary
Extract the Berthier Mle 1892 into your mod_overrides.
Make sure you have all the required mods.
Tactical Bolt Check is optional, but recommended to get the intended audio experience. If running Tactical Bolt Check, you may need to also update your Advanced Movement Standalone if you have a very old version of Advanced Movement.
Weapon Rename Tweak Data is optional, but recommended to automatically rename your Berthier variants by changing attachments, no typing needed.
Weapon Painter is needed to access the APX Scope's Accurate Reticle option.
Pawcio's Iron Sights pack and Custom Attachments for Custom Weapons are optional as well. You need both of them together to add Pawcio's iron sights to the Berthier.