Things You'll Need
Instructions
Place your Action Replay into the Nintendo DS cartridge slot for DS games, not Game Boy Advance games. Insert "Pokemon Diamond" into your Action Replay. Turn on the Nintendo DS.
Press the "Code Select" button, which looks like an asterisk. Tap a code on the DS' lower screen to activate the code. Press "Start" to launch "Pokemon Diamond" with your thief code enabled.
Play the game until you encounter a trainer battle. Wear their Pokemon down to only a few HP, as you would when catching a wild Pokemon. Throw a Pokeball at the trainer's Pokemon and, if the HP is low enough, you'll catch it.