I did a B2B on Horizon in October. You'll get a letter in your stateroom detailing the process.
With the letter was a pass to let you back on the ship if you opted to disembark with the other passengers and spend the day ashore.
The other option, which I chose, was to wait in the designated area (one of the MDRs) until the ship was empty of non-B2B passengers. There they distributed the Sail and Sign cards for the next cruise. When the ship was clear of non-B2B passengers, the staff led us off the ship where we scanned out with the old S&S card for the last time, went down to immigration and did the face scan, then waited in line until it was time to re-board (not very long). For a short while we had the ship to ourselves.
I took the opportunity to enjoy the Havana pool area, since I was not staying in a Havana cabin it was my only opportunity to check it out before the passengers arrived.
Have fun!