Haha, I don't think you need to be on any specific firmware for the method I used to install the custom firmware on my New 2DS. Mine is on the latest version.

If you search "3DS custom firmware" or Luma3DS there are a few great tutorials out there that should give you all the info you need. That includes what flash carts to use and all of the bits and pieces you need to get it done.

Hopefully that helps!