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.
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
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.