Here is a guide on getting the files on your already jtagged system and preparing it for hosting MW2 lobbies. Yes, this guide is assuming your system is already jtagged, if you need a guide for that check teh internets.
1)Install xexloader 0.26
2)Run XeXLoader and Create a folder on the root of the drive called "Games" then in that directory create another folder called "MW2" go into that folder and select "Copy DVD"
3)Replace original Files with modded ones in MW2 Directory(Make backups of originals first!)
the modded code_post_gfx_mp.ff I use can be found here
The patch for your can be found heredefault_mp.xex patcher. *Thanks to TheSheep who posted this*
**MAKE SURE YOU OVERWRITE THE CORRECT code_post_gfx_mp.ff and NOT code_post_gfx.ff THE FILENAMES ARE SO CLOSE AND THIS IS USUALLY HOW YOU GET THE DREADED DISK READ ERROR!***
**I strongly recommend putting your DVD Drive to stock firmware after you installed MW2**
4) Get the latest Title Updates from this site XBOX 360 Update Center and place them in "Cache" folder found on the root of the HDD, copy the files over with xexloader.
5)Reboot xbox with CAT5 cable, Log into live. If you have XBR3 it will ask to update, Dont be afraid and DO IT. I dont even have my R6T3 removed and it only updates the data you dont have, it will know you're already running 8955 and only add avatar support.***If you already have Avatars you most likely dont have to do this***
6)Load up xexloader and run MW2 from the menu, then go into Multiplayer and at that point you can connect to Live.
***If you have the original MW2 disk I would strongly recommend having it in the drive even though you arent playing from the dvd, just for live check I cant guarantee you wont be banned but you need all the luck you can get***
NOTE: For match hosting you want it to be Private>Free-for-All>Any Map You Want! To turn on the xp mod, at the beginning of every match you need to press "D-Pad, Up, Right, Down" during the countdown, you know it's working because the countdown restarts. You need to do this at the start of every match! The code-post-gfx-mp provided is a 2 kill based xp hack, 1st kill gets you rank 70, 2nd kill gets you that extra xp to prestige.
TIPS FOR HOSTERS! Never EVER use your actual gamertag, use one that will only last 2 days, then make a new one for 2 days, rinse repeat. Reason: It's been a week since I hosted and The gamertag I used is getting messages every minute STILL and it wont stop.