This method is no longer accessible due to the closure of the eShop. However, if Pokémon Picross was downloaded before closure, it is still usable.
Required Reading
Pichaxx is an exploit for the game Pokémon Picross released by zoogie, originally discovered and demonstrated by MrNbaYoh. This method of using Seedminer for further exploitation uses your movable.sed file to write a custom save file for Pokémon Picross, which can then be used to launch the Homebrew Launcher.
This process requires your console-specific movable.sed file. If you don't have it yet, you can get it by following seedminer. (Don't continue to BannerBomb3.)
This process will overwrite your Pokémon Picross save file, if you have one. If you wish to preserve your Pokémon Picross game data, you should make a backup of your 00000001.sav file before overwriting it.
What You Need
The game "Pokémon Picross" (free on eShop) installed on your device
You can scan this QR code using the Nintendo 3DS Camera for a direct link to the eShop app
Your SD card must be inserted in your device to install Pokémon Picross
Copy boot.firm and boot.3dsx from the Luma3DS .zip to the root of your SD card
The root of the SD card refers to the initial directory on your SD card where you can see the Nintendo 3DS folder, but are not inside of it
Copy the otherapp payload for your region/version from the unSAFE_MODE .zip's otherapps_with_CfgS folder to the root of your SD card and rename it to otherapp.bin