Once you have your product key and are on a PC running Windows 8.1, Windows 8 or Windows 7, follow these steps:
Run the Windows 8.1 Setup program. (
http://go.microsoft.com/fwlink/p/?linkid=271128)
When prompted to run or save the file, choose Run
When prompted to enter a product key, enter the 25-digit product key you received when you bought Windows 8.1.
When prompted to install Windows 8.1, select Install by creating media, then click Next.
Select the type of media you want to use: either a USB flash drive or a DVD.
To create a USB flash drive
Click on USB flash drive.
Plug the USB flash drive into a USB port on your PC.
When prompted, select the USB drive you want to use, then click Next.
If you have files on the USB flash drive, they'll be deleted.