Homebrew

Open any software (it could be literally anything, but usually you would want to use a software that isn't commonly used such as Health & Safety Information or Download Play).

Once the software has loaded, open the LumaCFW menu. (Default combination is L + D-Pad Down + Select, all at once.)

On the menu, select Miscellaneous options..., then press A on Switch the hb. title to the current app.

If it is successful, it should say Operation succeeded. Press B until you exit the menu, then go to the home menu and close the software.

Reopen the software. You should see the blue Homebrew menu. On the bottom screen, there is a file browser, which will list all software under the 3ds folder on the SD card.