For preexisting characters: In your game's "img" folder, navigate to "characters". Drop the replacements in.
For Generator: In your RPG Maker MV system folder (usually steam/steamapps/common/RPG Maker MV) you will see a folder named "Generator". This is the in-game generator. Inside is a "TV" folder with 3 bodytypes; Female, Male, Kid. This folder has all the parts for the overworld character (hair, clothes, etc).
In my pack, however, the bodytype folders contain a folder for every reface. Pick a face from the sample guide, and copy the 2 files. Paste these 2 files into the matching "TV" folder (e.g. if you want the female character to have the People16 face, go into Female/People16 and then copy those files into the Female folder of your generator).
You may notice this is extremely tedious for making a lot of sprites with different faces, since you'd have to do it over and over again, but this is how it's always been 🥲