Lenny's Mod Loader is an easy way to modify your game. In its current state for RDR 2, it will allow you to replace any file in the game, be it streaming files like model or data files, font files, etc. It also includes full UI support like in GTA 5.
Installation:
Copy all files from "ModLoader" to the root folder of the game so that files like vfs.asi are in the same folder as RDR2.exe.
The Mod Manager folder can be located anywhere and not necessarily inside the game folder.
New mods are placed in the lml folder or click "Download with Mod Manager" on the mod's website if it supports this installation option.