The image to the left is the final result (replacement of Gold Ore).

This replaces the gold texture and template is here:

MxSxC1/MCResPackTutorial: Minecraft Resource Pack Tutorial (github.com) 

Make sure your textures are 64*64 (or a multiple of). You do not need _n or _s unless you are doing PBR textures, which are not covered here (incoming).

Create this structure, change [USERNAME} to your username, or amend to suit your install location.

mxsxc1_goldore is name of the resource pack, can change to whatever you want.

C:\Users\[USERNAME]\AppData\Roaming\.minecraft\resourcepacks\mxsxc1_goldore\assets\minecraft\textures\block\

Save the following as a text file called pack.mcmeta (you will need to increment number if using later version but 99 works for current)

{

  "pack": {

    "pack_format": 99,

    "description": "MxSxC1GoldOre"

  }

}

Save it to this location:

C:\Users\[USERNAME]\AppData\Roaming\.minecraft\resourcepacks\mxsxc1_goldore\assets\pack.mcmeta

In this folder drop in the image you want to replace gold ore with and call it pack.png (optional)

C:\Users\[USERNAME]\AppData\Roaming\.minecraft\resourcepacks\mxsxc1_goldore\assets\pack.png

Copy same image into the following and call it gold_ore.png

C:\Users\[USERNAME]\AppData\Roaming\.minecraft\resourcepacks\mxsxc1_goldore\assets\minecraft\textures\block\gold_ore.png

Load the resource pack in game, and done...

As per screenshot on the left, set your new pack to "Selected" on the Resource Pack Screen.

If it is showing as non-compatible, you will need to change the number in pack.mcmeta to the latest version, to the latest number...it increases 1 with each version.

   "pack_format": 13,

See top for final result...and the replaced gold ore.