How to Create Bootable Windows USB Disk

https://anandthearchitect.com/2014/03/07/how-to-create-a-bootable-windows-2012-r2-usb-disk-applies-to-windows-8-1-also/

Pre-requisites: 7-Zip software (Download it from here: http://7-zip.org/), Windows 2012 (R2) ISO (or Windows 8.1 ISO), 8GB or more USB disk

Open Command Prompt in elevated mode (Run as Administrator)
Type diskpart and press Enter
Type list disk and press Enter. Note the list of existing disks.
Insert the USB Disk
Type list disk and press enter again. Note the new disk showed up which is our USB disk. I assume the new disk is 2 for example purpose.
Type select disk X where X is your USB disk. E.g., select disk 2. Press Enter.
Type clean and press enter.
Type create partition primary and press enter to create primary partition 1.
Type select partition 1 and press enter.
Type active and press enter to make the partition 1 active
Type format fs=ntfs and press enter. This will format the partition 1 as NTFS volume.
Type assign and press enter to assign the USB disk to a drive letter.

Now right click on Windows Server 2012 R2 or Windows 8.1 ISO file, select 7-Zip –> Extract Files…
Select your USB disk to extract the ISO contents to the USB disk