First of all your going to need a Xbox to put the pc into. Just open all of the screws from the bottom of the xbox and pull the top up from the console. Once the top is off, pull out the stuff from the inside including the metal frame.
Step 2: Power button
If you want to use the Xbox original on/off button, dvd disk opening button and the leds for the power on and the hdd youl have to modificate the Xbox button circuit. This includes some soldering but nothing too hard. First of all you need some wires for the two leds (each on the othed sides of the circuit.) the power and dvd opening button. The each led in the Xbox have two colors Green and Red. You can choose wich one you like. i decided to use green light for the power and red to the hdd. Just solder wires to the leds and for the power and dvd button. If you don't know how the buttons work use a multimeter to work that out.
Once you have solderd the wires to the circuit youl have to cut the copper from the black side of the circuit to make it work. just follow the red lines in the last picture and make sure that it wont lead between the cuts anymore. if you don't do this the buttons wont work.