"I want you to tell me—not me to tell you—HOW I'm DOING the transfer"
OK, I'll tell you just what to do.
Whether you take my advice or not... is up to you.
Plug the external drive you have into the old MBP.
It's going to be erased, so make sure there are no files of value on it when you do this.
Download SuperDuper by clicking this link:
SuperDuper is FREE to use for this purpose (this costs you nothing)
Use SuperDuper to "clone" the contents of the OLD MBP to the external drive.
It's one of the easiest-to-use Mac apps I've ever seen.
When the clone is done, disconnect the drive and take it to the new MBP.
IMPORTANT:
The best way to do a migration is to a "completely blank" NEW Mac.
That means you do the migration when you boot the new Mac for the very first time.
I'm thinking that you have already put an account on the new Mac, is this correct?
I think we need a clarification from you before proceeding further.