1- Make ISO file of windows disc or DVD with your favorite program/software.
2- Download the Windows 7 USB/DVD Download Tool from Microsoft and then install it. This free program from Microsoft does everything includes properly formats your flash drive and then copies the contents of the Windows 8 installation ISO file you have to that in USB drive.
3- Start the Windows 7 USB DVD Download Tool program. You should be able to find the shortcut on the Desktop or in your Start menu or on your Start screen.
4- Click Browse and Locate, and then select, your Windows 8 ISO file. Then click Open.
5- Click on Next.
6- Choose USB device on the Choose media type screen.
7- Insert USB device screen, choose from the drop-down box the flash drive or USB-connected external hard drive you want to put the Windows 8 setup files onto and then click Begin copying.
8- Click Erase USB Device if you’re prompted to do so on a Not Enough Free Space window. If you don’t see this, don’t worry, it just means that your flash drive or external hard disk is already empty.
9- Creating bootable USB device, just wait for the Windows 7 USB DVD Download Tool to prepare the drive and copy the Windows 8 installation files to it.
10- The next screen should say Bootable USB device created successfully with a Status that says Backup completed.
11- Boot from the USB device that you just created to start the Windows 8 install process.