In this lab I will use Rufus 2.2.668 to create a bootable Windows 8.1 USB drive for installation.
Launch Rufus 2.2.668, click the browse button to find the Windows 8.1 ISO image.
After opening the ISO file, enter a label for the USB drive and click Start.
Click OK to confirm that the USB drive will be formatted.
Click Close when done. Now, you can boot the USB and install Windows 8.1. Note that this method is much slower than the diskpart method that I introduced earlier: