Replacing iPhone and cellular Apple watch at the same time
I am about to replace my iPhone and cellular Apple watch with new devices, and I will be setting both of them up on the same day. Can you recommend the best way to transfer my current services to new devices? E.g., do I set up the watch first, or the phone, and how do I make sure I maintain NumberSync (and have it go to the right watch) and don't lose my existing lines, or accidentally add a new line to my account (and get charged extra for it) in the process.
Ideally, I am looking for a list of sequential steps, with links to support articles or other info for how to execute each step.