This is a blog post I wrote at my company web site MobilApp.nu (mobile app development)
It is aimed for our customers, who are mainly non-developers, on the steps for first time setup Apple Developer and App Store Connect for us to be able to sign and upload apps that we have developed for them. But it should be usable for any non-developer or developer in that same situation.
https://www.mobilapp.nu/2020/02/12/tutorial-add-your-app-to-apple-app-store-preparations-for-non-developers/