I'm in the same boat! Was expecting to have instructions in the box where the S9 came but nothing there. A couple of days ago got a reminder email from Samsung that they haven't yet received the old device. They had a link on the email to send a switching programme return pack, which I requested but so far haven't received nothing.
If they were better organized, having sent the S9 in such a big box, they could have included a label and intructions and use the same box to return the old phone. Lets see what happens.