While this is fresh in my head, I'll bash out these instructions. You'll need a USB thumb drive of 8Mb capacity, named "Untitled". It will be erased as part of the process so make sure it's either empty or contains nothing of value. Download macOS from the Mac App Store. It will auto boot, so stop it right away. If you allow it to install, it will disappear from Applications and you will need to download it again. Copy the following: sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app Open Terminal and paste the above into the screen. When you hit return, you'll be prompted for your account password.The password will not be visible in the Terminal window. Don't worry. That's normal. Hit return and you'll be warned that your drive will be erased. Type "Y" and return to continue. The installer files will be copied to the USB drive. To use it, boot your Mac while holding the "Option" key.