If you are on 11.0.0, you must follow this guide to downgrade your firmware using one of the methods at the bottom of this page to directly dump and restore your NAND.
Both of these are working implementation of the "FIRM partitions known-plaintext" exploit detailed here.
This will work on both a New and Old 3DS (although the DSiWare method has some restrictions).
Use this link to downgrade your Firmware with a hardmod
Use this link to downgrade your Firmware with DSiWare and a second 3DS (which already has CFW)
OTP Info
NAND Size
Geräteinformationen
Part 1 - Decrypt9
Part 2 - 2.1.0 ctrtransfer
Part 3 - arm9loaderhax
For support, ask for help at #3dshacks on Rizon IRC.
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