All Mac operating system software is time based i.e Sierra 10.12 was released 20th September 2016.
So before starting, your best changing the time, month, year on your mac via terminal or the system preferences. That way you won't get any error messages when your reinstalling an older osx version.
Secondly download a copy of Sierra and make a bootable usb pen drive.
Insert pen drive into usb, turn on machine, hold down ALT key immediately and the Sierra installer icon will appear onscreen.
Click it and go straight to Disk Utility and repartition back into HFS file format and install fresh copy.
Then once done take a fresh image back up onto Time Machine.