It's easy. You uopen up your xbox (Using the T-20 torx for the 6 screws on the bottom) you remove the HDD with a T-10 torx and the DVD drive, you pull out the motherboard using the T-10, and then you go to xbox-scene and look up a tutorial.
Here, I'll even do the work for you.
http://www.xbox-scene.com/tutorials.php?p=191|315|#234