Step 1: Step 1: Getting the Default Pack and Adding It to MC

So, I found a YouTube video, with all the download links from 1.8 all the way to 1.12.2:

And you can download the 1.12.2 original texture pack there. Put it in a place you can easily find. Then, you'll open the Windows Start Menu. There, you search %appdata% and hit enter. That opens a folder called Roaming. Then, click .minecraft, then click Resource Packs. Drag your .Zip file into Resource Packs, and open it. Then, click Minecraft, then Textures, you'll see that you have access to all the Items, Blocks, Entities, And even the splashes, it shows on the main menu!

Step 2: Editing, and Testing.

Next, extract it to a folder, and put the folder in place of the .Zip file. Now, open that, and edit WHATEVER you want. Once your done, open Minecraft. Go to Options, ten Resource Packs. Next click on the texture pack, and add it. You'll notice that the texture pack is RED, and claims it's not made for that update. As far as i know, that doesn't effect it. But be careful, you might have some troubles. As you'll see in the Pictures, I changed the Bow, Iron Sword, A couple of paintings, the arrows, and the pumpkin.

By the way, I've found that using 1.11.2 .zip Says its made for that update, but doesn't change the items.