There are several methods to installing custom firmware on the 3DSXL, but the simplest way involves installing the ntrboot exploit by way of a MicroSD-powered flash cartridge. This inexpensive piece of hardware will allow the 3DS to run ROMs from the aforementioned MicroSD card. In my opinion, the R4i Gold 3DS flash cart is the best on the market and will come with the fewest functional restrictions.
Before beginning, download the latest versions of boot9strap, SafeB9SInstaller, Luma3DS, and ntrboot_flasher_nds. You’ll also need a small magnet.
Flashing ntrboot to R4i Gold 3DS
Insert a blank MicroSD into your computer (operating system doesn’t matter as you’re only moving files to the MicroSD card). In the root folder of the MicroSD card, create a folder called ntrboot.
Unzip the boot9strap*.zip archive and copy the boot9strap_ntr.firm file into the ntrboot folder. Copy the ntrboot_flasher_nds.nds file to the root folder of the MicroSD card.
Eject the MicroSD card from your computer, reinsert it into the flashcart, insert the flashcart into the 3DS and power on the system.
Launch the flashcart from the 3DS main menu, then launch the ntrboot_flasher_nds.nds ROM from the flashcart menu. Follow the on-screen instructions to dump the flashcart’s memory for a backup. After making a backup, select “Inject FIRM” to install boot9strap on the flashcart. Once finished, power off the 3DS.