Requirements
Before installing, ensure you have the following tools set up in your GTA V directory:

Script Hook V

Script Hook V Dot Net 

OpenIV

Installation Guide

1. Installing the Script

Extract the downloaded mod archive.

Locate the RE4_HUD.cs file.

Drag and drop this file into the scripts folder located in your main Grand Theft Auto V directory.
(Note: If you don't have a scripts folder, simply create a new folder and name it scripts).

2. Installing the Textures (.ytd)

Open OpenIV and enable "Edit mode".

Navigate to the following path inside your mods folder:
mods \ update \ update.rpf \ x64 \ textures \ script_txds.rpf
(Note: If you don't see this path, navigate to the original update.rpf outside the mods folder and click "Copy to mods folder").

Drag and drop the re4_hud.ytd file into the script_txds.rpf archive.

Close OpenIV.