How would I do that? I have a DSTwo.
download the gateway software from their site,
open it up and go into the "blue card" folder and copy the "gatewayinstaller.nds file onto your DStwo,
go back out and go into the "gateway omega" folder, copy the "launcher.dat" file from here onto the big SD card that goes in the side of the 3DS
put both cards in the 3DS, run the DStwo and launch the "gatewayinstaller.nds" like any other ds rom, select your region and let it install, one it finishes it will automatically close DS mode, press the home button to be returned to the homescreen
now go to system setting -> other settings -> Profile -> DS profile, you will be taken to the gateway menu, just select classic mode and you will be able to run 4.x+ games from the home screen
(at this point there is no point setting up emunand as updating to 8.1 requires the gateway card to be inserted to enter classic mode or it will just hang on a black screen....so just stick to using classic mode on sysnand)