Once you receive your new phone. Transfer the data from the old phone to the new one, or however you wish to set the new phone up.
Normally in the packaging with the new phone, or it's sent separately. The return details and returns lable.
Be careful not to confuse the trade in returns lable with the returns label for the new phone if you needed to send that back.
You only have 7 Days, so sending it in as soon as possible is a good idea. They won't take into account things like any issues the postal service may have.
Sign out of your accounts on the old device and the factory reset it.
I take plenty of dated pictures to show the condition of the old phone and with the screen on as well.
If you have the old box the old phone came in, you can use this to put the old phone in and put it in the returns package.
Double check you are affixing the returns label to the package. It has a unique number on the lable specifically for your trade in.
Personally I'd always pay extra for sending it. One you get them to sign for the package and I get the insurance as well.
But if you want to just use the paid for lable, that's upto you.
A good idea that
@BandOfBrothers has is to film unboxing the new phone in case there are any issues. Something I might start doing as well.
With all that said. I've never had any problems sending in a trade in phone. But it's all just in case.