Selling my old iPhone 6, but I need to make sure all my photos and videos etc are saved before I delete them all, how do I make sure of this?

Selling my old iPhone 6, but I need to make sure all my photos and videos etc are saved before I delete them all, how do I make sure of this? - 1

Back them up.

Copy them to somewhere else, like a computer, a flash drive, online storage etc.

ITunes or icloud back up