Luma3DS forks are modifications of the Luma3DS custom firmware. These forks can:
Add features unavailable in upstream (unmodified) Luma3DS
Enhance existing features in upstream Luma3DS
Remove features deemed 'unnecessary' in upstream Luma3DS
There are some things that you should take note of when using Luma3DS forks:
They may not always be kept up to date with upstream Luma3DS, so you may miss out on any new features added to Luma3DS.
If you experience a bug with one of these forks, you must report it to the developer of that fork. The developers of Luma3DS are not responsible for any issues or bugs that may occur when using a fork unless the same issue also occurs in upstream Luma3DS.
If you're getting assistance with an issue, you should specify if you are using a Luma3DS fork, as troubleshooting may differ.
Installing a Luma3DS fork
These instructions assume you're using boot9strap (as installed in the guide) as opposed to fastboot3DS, where these instructions may not apply depending on your setup.
Because boot9strap just boots the file boot.firm on the root of your SD card (or on CTRNAND), you can just place any Luma3DS fork onto the root of your SD card as boot.firm. Your 3DS will automatically boot into it as it did upstream Luma3DS.
Download your Luma3DS fork of choice
If necessary, extract it and/or rename the file to boot.firm
Put your SD card into your computer
Copy the Luma3DS fork's boot.firm to the root of the SD card, replacing the existing one