This process is legal under fair use / right to backup in many countries, as you are extracting keys from hardware you own for personal emulation.
: Most "top" results (often found on GitHub or specialized emulation wikis) provide a ready-to-use text file. You simply drop it into the sysdata folder within your Citra directory. citra aes keystxt download top
: Ensure your 3DS is on firmware 11.16 or 11.17 and has GodMode9 installed. This process is legal under fair use /