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.
• Extracts the decrypted NAND to the chosen output destination (7-Zip required for Windows).
• 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 remove the No$GBA footer or add it.

• 7-Zip 19.00 or greater.

• Python 3.5 or greater with the Tk library (I had to do sudo apt-get install python3-tk -y in my Ubuntu virtual machine, sudo dnf install python3-tkinter in Fedora, sudo pacman -S tk in Arch Linux). You might need to install the Python 3 distutils package also.

• Python 3.5 or greater, you can install it with one of these options:
  • [Recommended] 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).
  • [Might have issues] The latest installer from the python.org releases page (3.7.3 at the time of this writing). NOTE: Using this option you might need to go to Applications > Python 3.<version> and run Install Certificates.command. This will avoid the "Could not get HiyaCFW" error.

• 7za binaries for 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 Linux and MacOS. Compiled from the WinterMute's GitHub repo. For Windows the twltool included with HiyaCFW is used.
• 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.
• fatcat binaries for Linux and MacOS. Compiled from the Gregwar's GitHub repo.

• Go to the helper's folder.
• Double-click on the HiyaCFW_Helper.exe file.

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