1 - Modern Warfare 3 - Reloaded

Obiously you can use your steam version as well
-Copy your /steamapps/modernwarfare 3 folder to a different location.
This way if something goes wrong you wont break anything.
Add files from Step 2 - Overwrite/merge
Download teknogods loader (step 3)

Or download:

thepiratebay.se/torrent/6804632/Call_of_Duty_Modern_Warfare_3-RELOADED

Move all files from Crack to install directory and overwrite/merge folders.