Here's explained in 4 steps -
1. Insert the 'original' game disk in your dvd drive
2. Download and install DVD Decrypter, search for it yourself on google
3. Install and open DVD Decrypter, go to MODE-ISO-READ
4. Select the drive where the image is to be saved.
5. it'll start creating an image of the game in .ISO format
6. When done, insert ONLY GOOD QUALITY (Like Sony's DVD-R for example) blank DVD-R media, i hope you know you can't do this without a good DVD Burner as well.
7. Open up DVD Decrypter again, go to Mode-ISO-Write, and select the ISO file you just created.
8. Uncheck Test Mode and Verify, they aren't really needed and just slow the procedure down.
9. Select the writing speed at
2X or LESS (Or else burning in fast speeds will only produce slow loading times and lags in the game)
10. Burn and play on your modded Xbox
That's the procedure i've been using for Ps2 for years without fail and i see no reason why it should not work for Xbox.
(@mods : Making backup copies of legaly bought original games is legal)