This page prepares consoles with an existing modern boot9strap installation for reinstallation and/or updating of custom firmware applications. It can also be used in the event of a lost or corrupted SD card.
Your SD card must be formatted as FAT32 to follow this guide, or else the 3DS will be unable to recognize it. If your SD card is not yet properly formatted, use one of these pages to format it, depending on your operating system: [Windows](formatting-sd-(windows), [Mac](formatting-sd-(mac), [Linux](formatting-sd-(linux).