This tutorial assumes that you have a compatible 3DS flashcart (such as the R4i Gold 3DS) that’s been flashed with ntrboot. Before beginning, download the latest versions of boot9strap, SafeB9SInstaller, Luma3DS, and ntrboot_flasher_nds. You’ll also need a small magnet. Insert the ntrboot-flashed flashcart and continue.
Installing boot9strap on the 3DS
Remove the MicroSD card from the 3DS console (not the flashcart) and insert it into your computer. Unzip the SafeB9SInstaller*.zip archive, then copy SafeB9SInstaller.firm to the root folder of the MicroSD card. Rename SafeB9SInstaller.firm as boot.firm.
Unzip the Luma3DS*.zip archive, then copy boot.3dsx to the root folder of the MicroSD card.
Create a folder called boot9strap in the root folder of the MicroSD card and copy boot9strap.firm and boot9strap.firm.sha from the aforementioned boot9strap archive to this folder.
Eject the MicroSD card from the computer and reinsert it in the 3DS console, then power on the 3DS. Use the magnet to locate the reed switch that controls the sleep function. This should be somewhere around the ABXY button cluster. Once the reed switch is located, power off the 3DS console and place the magnet over the reed switch. With the magnet in place, hold START + SELECT + X + POWER to boot into SafeB9SInstaller. Once all the startup checks complete, you can remove the magnet. Follow the on-screen instructions to install boot9strap, then, once the installation completes, hold the POWER button until the console turns off.
Once again, remove the MicroSD card from the 3DS console and insert it into your computer. Delete boot.firm from the root folder of the MicroSD card, then copy the boot.firm file from the Luma3DS archive to the root of the MicroSD card. Eject the MicroSD card from the computer and reinsert it into the 3DS.
Power on the 3DS and it will boot into the Luma3DS configuration menu. Enable “Show NAND or user string in System Settings” from the configuration menu. Press START to save and reboot.