Installation

Click the download button and choose the correct Minecraft version Launch your Minecraft instance and go to Options, then click the Resource Packs option. Click the button reading Open Pack Folder. Grab the downloaded file and put it in your \minecraft\resourcepacks\ folder. Select the pack in-game and hit Done. Ta-da! You're done!

How can I make one myself?

To make a custom panorama pack for yourself, you will probably first want a template pack. You can download mine and edit the contents to your liking.

Setting up the Pack

If using a template pack

Extract the .zip file into a new folder. I recommend you put it into a folder as the extracted files can easily be lost.
Open \assets\minecraft\textures\gui\title\background\ and you will see all 6 images labeled panorama_0.png, panorama_1.png, panorama_2.png, panorama_3.png, panorama_4.png, and panorama_5.png.
Delete all 6 of these .png files.

If not using a template pack

Create a new folder named assets, and 5 more folders that will go inside. The final folder's file directory should look like this:

assets/
   └─ minecraft/
      └─ textures/
         └─ gui/
            └─ title/
               └─ background

Create a pack.mcmeta file that will go in the final .zip alongside assets
This pack.mcmeta's contents should consist of:

{
    "pack": {
        "description": "Custom Panorama",
        "pack_format": 34
    }
}

"pack_format": 34 <- This value depends on your Minecraft version. You can check the proper pack_format for your version on the Minecraft wiki.

├─ pack.mcmeta
└─ assets/
   └─ minecraft/
      └─ textures/
         └─ gui/
            └─ title/
               └─ background/
                  ├─ panorama_0.png
                  ├─ panorama_1.png
                  ├─ panorama_2.png
                  ├─ panorama_3.png
                  ├─ panorama_4.png
                  └─ panorama_5.png

(You can add a pack.png that will be the pack's in-game icon. Make sure the aspect ratio is 1:1)

Getting the Images

Create a fresh Minecraft instance on Minecraft (Java) version 1.21.9+ (I do not recommend doing this on an existing instance as it uses debug features added in 1.21.9 that are experimental.)
Add these Java arguments to your launcher:

-DMC_DEBUG_ENABLED -DMC_DEBUG_PANORAMA_SCREENSHOT

In the official Minecraft Launcher, go to Installations -> [Your Version] -> Edit -> More Options, and paste the arguments into the JVM Arguments field, before any existing text. If you're using MultiMC or Prism Launcher, edit the instance settings and add them under Java -> JVM Arguments.
Launch your instance, and open the world you would like to use.
Click Ctrl + F2. If you are in the air, I recommend you hold Space as well to prevent moving downwards.
After pressing Ctrl + F2, Minecraft will display a chat message like "Saved screenshot as ...". Click the underlined text to open the screenshots folder inside your Minecraft instance directory. You should find six images named panorama_0.png through panorama_5.png. Drag and drop these into the background/ folder you created earlier.

If you don't see all six images, make sure the JVM arguments were added properly and that you pressed Ctrl + F2. You might need to relaunch the world and try again.

Finishing your Pack

Right-click and compress your pack.mcmeta, assets, and your (optional) pack.png files into a .zip file with any name you desire. You can use this pack for personal use, or share it with others!

Make sure you zip the contents (not the folder containing them) so that pack.mcmeta is at the root of the .zip, not inside another subfolder.

The pack itself took me ~30 minutes to make. On the other hand, making the description took me around 1 ½ hours 😭