Here’s a concise, informative review of the process for downloading and installing a NAND backup on a 3DS.
sudo dd if=path/to/nand.bin of=/dev/sdX bs=1M status=progress
sync
If you're looking to install CFW, here's a simplified overview: 3ds nand download install
Step-by-Step Guide to 3DS NAND Download Install Here’s a concise, informative review of the process
Launch GodMode9: Hold the (Start) button while powering on your console. On Linux/macOS: sudo dd if=path/to/nand