The process varies a lot depending on the computer that you are using. Traditionally, you would open the BIOS menu and change the order of drives in the BOOT menu. Newer computers use UEFI instead of BIOS, which needs to be accessed in a special way if you're using Windows 8.
Reboot your computer, and look for the BIOS or BOOT menu key when the manufacturer’s logo appears. Press the setup key to open the menu and change your boot order. The setup key is typically F2, F10, F11, or Del. Make sure that the USB is listed as the primary boot device.
Then you’ll see a screen saying: ‘Press any key to boot from CD or DVD ....’
Wait for the Windows 8 installation files to load.
You'll know that the Windows 8 setup process is starting properly if you see the Windows 8 splash screen as shown above.
Choose language, time, and other preferences.
Click Install Now
Wait for Windows 8 setup to begin.
Enter your Windows 8 Product Key
Accept the Windows 8 Software License Agreement
Choose the Custom Installation Method
Bring up the Windows 8 Advanced Drive Options
Delete the partition you plan on installing Windows 8 onto.
Confirm the partition deletion
Delete other partitions used by the previous operating system.
Confirm Other Partition Deletions
Choose a Physical Location to Install Windows 8
Wait While Windows 8 is Installed
Restart Your Computer
Wait for Windows 8 Setup to Begin Again
Wait for the Windows 8 Basics Wizard to start
Follow the next steps, and your clean installation of Windows 8 will be complete!