Instructions
Insert your flash drive into a free USB port on your PC and click "Start," "Computer," and double-click on the drive.
Click "Start," "Internet Explorer." Visit the Microsoft "Get Updates for Your Xbox 360 Console" webpage. Click the "Here" link under "Update using a USB flash drive" to download and save the update file to a convenient location on your hard drive.
Right-click on the file once it's fully downloaded and click "Extract All."
Click and drag the extracted folder named "$SystemUpdate9199" into your flash drive folder to copy it to the drive, and then click the "Safely remove" button in the taskbar. Pull the flash drive out of your computer.
Turn off your Xbox 360 (if it's currently on) and insert the flash drive into a USB port on the console. Turn the console on, and then click "Yes" when prompted to begin the flashing process.