Ok first you will need two things.
1.A usb flash drive
2.A patch_mp from your desktop.

------------------------------------------------------

In this tutorial i will be telling you the way i do it

First i will my USB drive by going to My computer,And then go to the usb device located in my computer.
Then i drag the patch_mp from my desktop and then put it in the usb root folder.

Easy right?

Then next i will turn on my jtag console and go to XeX Menu.
I will go to USB and highlight the patch_mp.
After you highlight it press Y and go down to copy.
After you copy there should be a thing in the top screen that says USB/COPY/patch_mp
When you see that then you know you copied it.
After that you will click the right bumper until you find HDD1.
Then you will want to go to games and look for your MW2/Cod4 ETC folder and paste the patch_mp
If it says destination already exists then thats ok.
After you do that go to your default_mp.xex located in your folder and click that.
It should bring you to the multiplayer screen and then you can have fun using the patch