We have done half a dozen BtB cruises. For what it is worth, here are our experiences:
1. The process on each turnaround day has been different - no two the same.
2. We have been mustered ashore for a number of hours and at the other extreme we have spent a few minutes in a bar to get our new identification.
3. Read and head the letter you get with care, but be ready to have the actual process vary from what was written. Regardless of what the letter says, bring something to amuse yourself with for a couple of hours. Life happens...
4. Find out in advance what the ship's policy is about in-transit passengers accessing their cabin when new passengers are not allowed into their cabins. If they allow you through the doors, try to do so when new passengers aren't seeing you. Or have your in-transit pass in your hand and your "elevator speech" about why it is OK ready. If they don't allow you through the doors, then carefully plan when you are going to leave your cabin after the ship is cleared.
5. If your ship has passenger laundries and you wish to use them, turnaround day can be a good time.
6. If you want to be first in line to sign up for limited availability activities, this is your chance.
7. Some ships have a general announcement to crew right before new passengers are allowed to board. Listen if you get a chance - it can be interesting.
8. Finally, we have had nothing but good vibes from the crew when we were BtB. Every BtB passenger makes their lives just a little bit easier.