The process to format an SD card to FAT32 is different for Windows, Linux and macOS.
ChromeOS is not an option and formatting an SD card with its built-in formatter may result in it becoming unusable by consoles.
Choose your operating system
Optionally, if you have a 3DS with custom firmware, you can instead use GodMode9:
If you already understand how to use the tools in the above guides, here are direct links to their pages:
- Windows: SD Memory Card Formatter + guiformat
- Mac: Disk Utility
- Linux: GParted + dosfstools