You can download the windows 7 usb/dvd download tool from http://www.microsoftstore.com/store/msstore/html/pbPage.Help_Win7_usbdvd_dwnTool location. then click “windows 7 usb/dvd download tool” location.
Then install windows 7 usb/dvd download tool by clicking the EXE
Once you run this Application you can see the below mentioned Window and select the Win 8 ISO file and click next.
Next window you can select the media type.
Next window you can select the USB device that you want to install Windows 8.
Once this finished you have a bootable win8 USB flash drive with you..