Traded my old watch for a upgraded version

I need to get my series 7 watch ready to trade in for the series 9 watch. I already have the upgraded watch so now I just have to erase everything from the series 7 so it can be shipped to my phone company as part of my trade in deal. I have a basic understanding of how to do this but I wanted to make sure I didn’t miss any steps. And looking for any step-by-step instructions on the Apple website is extremely frustrating and feels very much like Clickbait. I just need clear instructions on how to do this not a bunch of information that I don’t really care about I just want cut and dry, clear step-by-step instructions on how to prepare my watch for the return.