Instructions
Start up the game and connect your modded controller to your console, if you haven't already. Just like a normal controller, press the wireless connect buttons on the controller and console to connect them.
Select "Multiplayer" from the "Call of Duty: Modern Warfare 2" main menu and then join a match.
Switch the lever on the bottom of the modded controller to "Black Ops/Modern Warfare 2" and then pull the right trigger to shoot. Your gun will now fire very quickly until you release the trigger. If your controller has two modded triggers, switch to dual-wield weapons in the game and then pull either trigger to use the modded controller.