Use the Media Creation Tool to download and save the .iso. Do not use it to make the USB. This saves you wasting time downloading Windows 8.1 and then having to redownload as the tool made the USB incorrectly. Use Rufus to make the Bootable USB. Its best to use a 8-32 GB USB flash drive and format as FAT32 (64 GB will be truncated to 32 GB for FAT32).

If you are installing in UEFI with SecureBoot (Newer Security Technologies - Systems made in 2012 or later, Systems made in 2010-2011 may also have the UEFI BIOS with the latest BIOS update but the settings will need to be enabled) use the 'GPT partition scheme for UEFI Computer'. Make sure to format as FAT32 or the USB Flash Drive will not boot.

This bootable USB drive creation tool is made by Microsoft. Although it is known as Windows 7 USB/DVD Download Tool, it works for other versions of Windows also. This bootable USB drive creation tool is made by Microsoft. Although it is known as Windows 7 USB/DVD Download Tool, it works for other versions of Windows also.

If you are installing in a legacy BIOS systems for systems without a UEFI BIOS then use the 'MBR partition scheme for BIOS Computer'. Its also recommended to format the USB Flash Drive as FAT32 but not necessary for a Legacy BIOS.

See here where I demonstrate the procedure:

Windows 7 Media Creation Tool For Windows 10

http://dellwindowsreinstallationguide.com/download-microsoft-windows-and-office/download-microsoft-windows/download-windows-8-1-retail-and-oem-iso/