You should already have Luma3DS installed on your SD Card, but if not, you can download the latest release here: https://github.com/LumaTeam/Luma3DS/releases
Once you have the randomizer files and Luma3DS on your SD Card in the proper folders, open Luma3DS by holding SELECT when turning on your 3DS. Check Enable game patching to apply the mod.
Enable Game Patching in Luma3DS
Once it is enabled, save and return to the Home Menu, then launch the game. To disable the mod, re-enter the Luma3DS menu and uncheck the option.