This is the same way as shown in previous post How to Create Windows 7 Bootable USB. But I posted it separately for How to Create Windows 8.1 Bootable USB. Creating Windows 8 bootable USB is very simple way using trusted "Windows7-USB-DVD-tool.exe" from Microsoft Store.

There are many Software that can be used to Create Windows 8 Bootable USB, but this one really reliable because it is made specifically for microsoftstore.com. but if you want to use Rufus to make bootable USB go here Create bootable USB using Rufus v1.4.3.I also try to create Windows 8.1 Bootable USB using Rufus and it works too.

Requirements to make a Windows Bootable USB:

• Microsoft Net Framework 3.5 if you don't have download it here.
  
• Windows 7 USB/DVD Tools Download it here.
  
• USB Flash Drive 4 Gigabytes or Higher.
  
• A Copy of any Microsoft Windows 8.1 Disk Image ISO File.
  

Step by Step Tutorial on How to Create Bootable USB:

• Install Microsoft Net Framework 3.5, if you Already have just skip this.
  
• Install Windows7-USB-DVD-tool.exe
  
• Insert your USB Flash Drive to the USB Port.
  
• Go to Desktop and Double click Windows7 USB DVD tool icon.
  
• On Windows7 USB DVD tool click Browse.
  
• Navigate to your Microsoft Windows 8.1 ISO then Click Open.
  
• After Open it Click Next.
  
• Click USB Device button.
  
• On the Dropdown menu select your USB Flash Drive.After you choose your USB Click "Begin Copying"
  
• Your Windows image is now begin to copying to your USB, it takes several minutes to copying. Just wait until it Finish.
  
• After Bootable USB Device Created, Click "Start Over".
  
• You may now Close Windows7 USB DVD tool then uplug your USB.
  
• You have now Windows 8.1 Bootable USB, You can use it to format your PC. On your Computer Bios, make sure your 1st boot is Selected to boot from USB. If you don't know how to install it. I refer you to this page How to Install Windows 8 or Windows 8.1.