WARNING:Taking apart Xbox WILL VOID YOUR WARRANTY!
1. First of all, you will need some tools:
Torx 20 and Torx 10 Screw Driver
2. Now on the bottom of the Xbox there are 6 points where there are screws you must remove
4 are under the rubber pads on the corners and 2 are under the barcode sticker and warnings. Using the Torx 20 screwdriver, remove these screws, from this point on, you are VOIDING YOUR WARRANTY, if you have one.
3.now you can turn the xbox over again and slip the Xbox top where there are ridges and pull straight up.
4. You should now be able to see the inside of your Xbox. You can see the hardrive, the disk drive, and the power cord and IDE cable

There are three more screws you must remove to completely take apart your xbox. Using the Torx 10 Screw Driver, remove these screws.
Now once again you can pull the whole platform that the hard drive rests on and take it straight out of the xbox. Careful as the IDE cable and Power cord for the hard drive is still connected, removing these before hand may help but if you are careful enough they should be fine.
Now you can see the motherboard of the Xbox. If you have a mod chip ready, get to soldering, and reconnect your xbox and get back to gaming!