Well I'm new to 3ds hacking so I'll tell you what I know and it could possibly help.
So it looks like you were trying to downgrade to 2.1 and you got softbricked? Before you do anything, try taking out your SD card and turning on your system. If it boots up, then just put the put card back in and continue with your tutorial. (And pls don't follow a video guide)
Also, according to other people, NANDS are attached to your system specifically. Using someone else's nand.bin will cause you to actually brick.
If the SD card thing doesn't work and you can't update your system using recovery mode then.. *shrugs* I'm a noob at this too. Anyways goodluck I'm sure someone else who knows more about this can help if taking out the SD card doesn't work.