Normmatt has created a way to run B9S .firm files from bootrom via a DSi Flashcard and a magnet! This works on every 3DS on any firmware version.
For installation without a PC, GBAtemp user @TheCyberQuake has created a pack which will automatically install B9S and copy over essential starter homebrew from the flashcard's SD to the 3DS's. This will mainly be used for PC-less B9S installations. If you have a PC with you, use 3ds.guide. Read more here.
This works because of a flaw in the bootrom. Before the bootrom boots the NAND, it checks to see if Start+Select+X is held down, and if the shell is closed. If these requirements are met, it will boot an NDS cartridge from the bootrom. This gives that cartridge bootrom access. You might be wondering how you'd hold down buttons while the shell is closed, and why you need a magnet. If you put a magnet in a specific spot on the 3DS, it will go into sleep mode. Using this, you can boot the NDS cartridge with the buttons held down while in sleep mode! Using a compatible flashcard, you can boot B9SInstaller using the flashcard, and easily install it on your 3DS. The 2DS does not require a magnet, as it uses a switch to sleep instead.
Q: Can Nintendo patch this?
A: Nope! Not without a new hardware revision.
Q: My flashcard is blocked by my firmware! Can I still use this?
A: Yup, the flashcard blacklist is not enabled on the bootrom.
Q: Why can't this work with my flashcard?
A: The installation requires you to flash to the flashcard's internal NAND. Most DS Flashcards have ROMs instead of NANDS.
Q: But my flashcard has a NAND! Why won't this work!
A: You can't just easily flash them, it takes time and effort. Please do not pester the devs for a release date for your card. It will be done when it is done.
Q: Do you know what flashcards are going to be supported in the future?
A: R4 SDHC Dualcore and the DSTT. Don't expect a release date, however.
Q: Will my 3DS flashcard work?
A: Sorry, the bootrom requires it to be a TWL (NDS) Cartridge
Q: I'm bricked! Can I unbrick it?
A: Yup! You can unbrick from any kind of brick as long as it isn't an MCU brick or you've taken a knife to the motherboard. You can even use this to boot EmuNAND on consoles with their NAND chip fried or even physically removed!
Q: Can I install B9S on the latest firmware with this?
A: Since the card has direct access to the bootrom, yes!
Q: Does this work on the New Nintendo 2DS XL?