What to do and how to apply the mods?

First you have to download modio. You can find it here at http://www.game-tuts.com/community/modio.php
Then have a look at the home page.
Now this is the IMPORTANT  part. Go onto your XBOX 360 (any version) and go to System Settings > Memory > Hard drive > Games. Now say if you wanted to mod Call Of Duty: World At War. Go onto that game and where it has the campaign where you should of started a new campaign then move that onto you Removable Disk (Memory Stick) and then take it out and plug into your computer. Now go onto Modio and click "Open From A Device" and click through the folders until it comes up with savegame.svg and double click it. Now when that opens you need to download a gamesave from Modio. Then copy the id from the non-modded and the thingy below it and make the modded one exactly the same as the other one and click save to device. Now once you have saved it then plug your Memory stick into your XBOX and go onto System Settings > Memory > USB. Then go onto Games then click Call Of Duty: World At War and move the modded file onto the Hard Drive and go back onto Call Of Duty: World At War and once it loads up what to play on click your Hard Drive and go to campaign and resume and a zombies match will start. The Video below is one of mine showing you how to download a SaveGame of Modio.