Things you will need:

USB Flash Drive
Modio
MW2 DVAR List


Here's what you do:

Put a MW2 gamesave on your USB Device
Open up Modio
Go to "Explore a Device"
Go to "File" then "Open/Close Drive"
Then go to "Content" > "Your Profile" > "Modern Warfare 2 (41560817)" > then "Game Saves (00000001)"
You should see a file named "savegame.svg" right click on it and click "Extract"
Save it to your desktop
Click the "Open a Save" on Modio and click on the save game file
Click "Mod File"
Click on whatever mods you want
Go to "Custom" and click "Load Custom DVAR List" and select the file that you downloaded
Click "Save"
Close the editor and click "Rehash and Resign" just to be safe
Open the "Device Explorer" back up and right click on the old MW2 save
Click "Insert" and Select the new save on your Desktop
***When it asks you if you want to overwrite it, click "Yes"

Go back onto MW2, Select the USB Flash Drive as the Save Device, go to "Resume Campaign", Have Fun!

**For Special Ops

Follow the instruction for modding the campaign
After you "Resume Campaign", "Save & Quit"
Go to "Special Ops" > "Solo" Choose an level and have fun!
*For online Special Ops Mods, Both player MUST have the EXACT same mods for it to work