In usbgx:
Select your game
Options
Ocarina menu > download.
If it fails then no cheats exist for this game.
If it works, you see a list of possible cheats, enable the ones you want and generate the ''gct'' (it is the file with codes that will be user by the game).
To use the ''gct'' file, go to game option and enable the ocarina option.
Note:
Some codes require édition of the code list on a computer before generating the gct file.