Download this cheat file:
http://cheats.gbatemp.net/Temp/files/TEMP_USRCHEAT.DAT.zip
And put the usrcheat.dat file in the "_dstwo" folder.
Now all you do is go to a game you want, highlight it, and press the X button.
There should be a menu for this specific game. In the "Cheat" area, make sure it says "on" and not "off" (you can change it by pressing the left and right buttons)
After that, press the "Cheat(Y)" icon (or press the Y button) and you'll have a list of available codes.
Just tick all the codes you wanna use (by pressing the A button or the touch screen), and then press the B button to get out of the Cheat menu, and press the "OK(A)" icon (or the A Button).
Enter the game, and make sure the cheats work.
You can disable them by going back to the cheat menu, and unticking them (or just touch the "Clear all" icon, which will untick all the codes together).
Of course you can use the cheats in real time, depends on what you set the hot keys to be (defult is Start+L+R iirc).
You can enter this menu any time you want (in-game), after entering it, press on "Cheat code"
and just pick the codes you'd like to use.