Formatting an SD card

The process to format an SD card to FAT32 is different for each operating system.

Optionally, if you have a 3DS with custom firmware, you can instead use GodMode9:
 * 3DS:GodMode9 Usage

If you already understand how to use the tools in the above guides, here are direct links to their pages:
 * Windows: SD Association Formatter + guiformat
 * Mac: Disk Utility
 * Linux: GParted + dosfstools