you have to solder in a Mod Chip and post fix, read the Nand from your Xbox360, flash xell to your xbox360 nand, write down the CPU Key, create an RGH Image and flash it to your xbox360 Nand.
There are Guides that explains it more in Detail but have in mind that if you never moded a console read carefully and follow the Guide step by step, there is a high risk to brick your console if not done properly.
You also require good soldering skills.
Good Luck