HOW TO Clean install macOS >
1 Back up your Mac
Before reformatting your Mac’s boot drive, you should back it up with Time Machine, just in case anything goes wrong. Plug in an external drive, go to System Preferences > Time Machine and back up in the usual way.
2Back up data
Check your Mac for documents, photos, videos and other files you don’t wish to lose, and back them up too. If your Mac was languishing in an attic or cupboard there probably won’t be any, but check anyway.