Things you will need for this method

Games this method works with
MW2
COD 4
COD World At War
Grand theft auto 4
Grand Theft auto 5

A computer
A PS3
A Brain
A USB
FIrst Step:
What you need to do first is FORMAT your ps3 (Unless you have lots of time and a USB that can hold everything on your ps3!)
Second Step:
You want to create a BACKUP (System Settings-->Backup Utility-->Back UP)
Third Step:
Head over to your computer
Fourth Step:
You will need to download the following programs
[url=PeXploit]http://www.thetechgame.org/forums/ps3-cheats-customization/809395-pexploit-lite-1-0-ofw-4-70-no-idps-needed.html[/url]
[url=PKGViewer]http://psx-scene.com/forums/content/pkgview-v1-3-ps3-psx-psp-pkg-viewer-extractor-2117/[/url]
Fifth Step:
Find the .PKG update for the game you want to mod
Sixth Step:
COD: Once you've got a clean .PKG for your cod game you will need to open it with PKGview. You will extract it. Now you will need to find a modded patch (I get mine from NGU) For mw2 you put the patch.ff where userdir is in the extracted pkg. That's pretty much all for this step. (You can skip gta if your not modding it)
GTA: For gta its basically the same process except the modded file is named differently for both 4 and 5. Im not sure what its called for 5 but for 4 its called updateimage (Or some where along those lines.)
Seventh Step:
Now that you got the modded patch in the extracted patch you will now open up pexploit
Once opened you will select your backup you made earlier.
After that where it says load packages you will select the extracted PKG you modified earlier (Make sure extracted Package is selected at the top)
Once you've selected all of your patches are selected click Patch and go.
That's pretty much all for the computer part
Eighth Step:
Once pexploit is finished remove your usb and go back to your ps3.
Now your gonna restore the backup. (System Settings-->Back Utility-->Restore-->Select your backup
Depending on how big your backup is it can take a couple seconds or hours!