I will not be liable for any damage of any kind
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You will need cfw and FBI
I'm just going to explain how to install the custom fonts, you have to get them yourself, search in google for it.
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Here a video how to make and install etc. custom fonts:

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So after you have your cia file (your custom font/s) you have to start fbi go in your cias folder and install the file after which you did this, you will need to close fbi and reboot your 3ds. Congratulations you have your own font on your 3ds.
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I tried it on my old 3ds xl.
I hope i could help you

 

[Deleted member 383179]

https://gbatemp.net/threads/tutorial-customising-a-system-font.477415/

This already exists and is posted already, but with more information.