Written Tutorial:

1.Ok first you gonna need a JTAG and RETAIL XBOX duhhh

2.Ok Now u want to get both xbox connected to the same router/modem [NOT TO EACH OTHER BUT TO THE ROUTER], You Can Either do this By a Ethernet Cable or a Wireless Adaptor or if you have a XBOX Slim you can use the built in Wireless.

3.Then Your Gonna Need to get the connected to TVs YOu can use 1Tv but using 2 is Easier
If you use 1 Tv its easier if you use different connection Methods EG 1 xbox HDMI , and the other component Cable and just switch channels to set up or just swap/change the cable if your tv dosent support different inputs.

The In-Game Setup

ON YOUR JTAG
1 You Need to Start Up like You Usually Do and Pick Your Patch You want to Use and load Defult_mp.XEX

2. Then You Need to SetUp a SystemLink Game Choose Any Settings/Map you like but if your on MW2 You need to make sure XBOX UNLOCKS IS ON and not off otherwise it wont work, but you dont need to worry about that on cod4/W@W.

3. Then Start your game and if your using a patch with a Verification System You Might Find It easier if on your jtag you make a profile with the GT you are Modding.


NOW ON YOUR RETAIL

1, Sign into live with the GT you want to mod then play the game you Jtags is running.

2, Then go to Multiplayer and go SystemLink

3, DONT create a game, Press JOIN A GAME and you should see your able to join your Jtags Game {If it dosent show straight away press X a couple times to refresh and it should then Come up, if it dosent you have done it wrong

3, Now when your joined If it says waiting to be verifed switch over to your Jtag and verify yourself but if you are using the same GT you shouldn't need to verify yourself.

4, Now you can do whatever you need to and when you leave on your retail go on to xbox Live as if your going to find a game and check everthing you need done has worked

Thanks, +Rep if it helped

Credit goes to me for writing this tutorial.