The advantage to following this process and creating bootable media is that it allows you to install Windows 8 without having to first run an existing operating system on your machine. If you change the boot order of drives in your computer’s BIOS, you can run the Windows 8 installation directly from your USB flash drive or from a DVD when you first turn your computer on.
1. Download Windows 8 Developer Preview
2. Download Windows 7 USB/DVD Download tool & install it.
Now open Windows 7 USB/DVD tool to prepare Windows 8 bootable USB flash drive. Specify the location of your Windows 8 ISO image & click next.
Select “USB Device” option,
select the USB drive which you want to convert into bootable Windows 8 drive and click Begin copying.
click on Erase USB Device to format it.
Click Yes to Continue,
It will start copying the files and after some time your USB will be ready.
Now restart your computer and boot from USB drive to begin installation of Windows 8. For Windows 8 installation step check this guide..