In order to make things easier for me, and because I don't have Windows, I edited the HiyaCFW Helper Python script by jerbear64 and LmN in order to add a graphical user interface to it.

• Allows you to browse for your NAND backup, no need to place it at the same folder.
• Shows the option to choose the output destination, which should be a (preferably empty) FAT formatted SD card or any other folder.
• Downloads the latest HiyaCFW release and decompress it.
• Autodetects the console region from the NAND dump, downloads and decrypts its v512 launcher.
• Creates the patched 00000002.app and bootloader.nds for the custom firmware.
• Uses your platform's twltool (binaries for Linux and MacOS included) to decrypt the NAND.
• Mounts the decrypted NAND (OSFMount required for Windows) and extracts it to the chosen output destination.
• Installs the HiyaCFW and the patched files on the chosen output destination.
• (Optional) installs the latest release of TWiLightMenu++ on the chosen output destination.

Clicking on the integrated circuit button will give you a NAND mode, where you can uninstall unlaunch or install its v1.4 stable release, remove the No$GBA footer or add it. Recommended only for those with a hardmod.

• OSFMount.
• You will need to run the HiyaCFW_Helper.exe file as administrator in order to mount the decrypted NAND.

• Python >3.5 with the Tk library (I had to do sudo apt-get install python3-tk -y in my LUbuntu virtual machine; sudo dnf install python3-tkinter in Fedora).
• You will need to run the script as sudo in order to mount the decrypted NAND.

• Python >3.5, you can install it with homebrew (install homebrew by running /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)" in a terminal window, then brew install python).
• Go to Applications > Python 3.<version> and run Install Certificates.command. This will avoid the "Could not get HiyaCFW" error.

• 7za binaries for Windows, Linux and MacOS. It's used to decompress the HiyaCFW latest release as @RocketRobz uploaded it as a 7z archive. Compiled from the kornelski's GitHub repo.
• twltool binaries for Windows, Linux and MacOS. Compiled from the WinterMute's GitHub repo.
• NDS bootloader creator binaries for Linux and MacOS (based off devkitPro's ndstool v1.27). Compiled from my GitHub repo. For Windows the ndstool included with HiyaCFW is used.
• fatattr binary for Linux. It's used for setting FAT attributes when writing to the FAT formatted SD card or in NAND mode. Compiled from the Terseus' GitHub repo.

• Go to the helper's folder.
• Right-click on the HiyaCFW_Helper.exe file and click Run as administrator.

• Open a terminal.
• cd to the helper's folder (cd ~/Downloads/HiyaCFW-Helper or whatever).
• Run sudo ./HiyaCFW_Helper.py.

• Open a Terminal (⌘+Space and write terminal).
• cd to the helper's folder (cd ~/Downloads/HiyaCFW-Helper or whatever).
• Run ./HiyaCFW_Helper.py.

Thanks to:

• jerbear64 and LmN for the original script.
• @RocketRobz for his HiyaCFW fork, its releases and for having the helper script on his repo.
• @Sha8q for the idea.
• WB3000 for his NUS Downloader source code.

Download it from the releases page.