As this has been asked about a few times, here's a guide. I'm writing this so I can link to it in the main pinned thread for reference. Note that this is only for copying a backup - You cannot copy an original 360 game like this!
I'm using ImgBurn for this, but you should be able to use other software that's capable of ripping discs to ISOs.
1. Insert your disc and fire up ImgBurn. If you use EZ-Mode Picker in ImgBurn, select "Create image file from disc" otherwise just go to Mode at the top, then select Read.
2. On the left-hand side under where it says "Destination", click the little folder/magnifying glass icon and browse to where you want to save your ISO and name it (what you name it doesn't matter), then hit Save.
3. Hit the big button at the bottom-left and the process will begin.
4. Once it's complete, run your ISO through the latest version of abgx360 which will create a .DVD file for you and check that the ISO is properly patched.
5. If everything checks out in abgx360, burn as normal.
