i use both NTR and arcode.
to use NTR, first look up the game, not all games are supported but some members here make codes.
sites to look up NTR code files, NTR codes are in PLG format.
http://www.elotrolado.net/wiki/Cheats_para_NTR
http://www.speedfly.cn/
(none of the sites in english, but just use google translate)
get the game id from the release notes for the game, then make a folder with the game id and place the cheat files inside, they should go on the root of your sd card in your plugin folder.
sounds hard but its not, go to site click on the codes you want, then place the cheat files in your plug in folders, each game needs it own folder.
boot the NTR (it will be on your homescreen and run it like you would any other game, you get a black screen and it will hack your system when it finishes it will say finished home button), you then load up any game either retail or games with games you installed with FBI, screen will flash green for a second after loading a game that tells you that the game is hacked, hit Y+X it will bring up the cheat menu, you then click on the codes you want to activate, some games however require you to do button combinations like start+1 etc... which should be listed on the cheat webpages.
I also use arcode,
http://www.fort42.com/ar3ds/game176/
lists the codes.