We have tickets but the site didn't say where to enter the building. I saw a similar question here recently but no longer find the helpful answer that someone had posted. If anyone else can speak to this, and how to get in the right line/door, that would be most helpful!
You enter at the main entrance with everyone else. The main entrance is on the water side. If you have the itineraries tickets, you show it to the ticket taker and meet in the courtyard for your tour guide.
Use the main entrance and ask where to meet your tour guide. My group assembled inside and not in the courtyard, so the location might vary.
Of the 10+ times I’ve been on this tour we’ve always assembled in the courtyard on the benches marked for tour meeting point. The entrance is on the side directly facing the lagoon and facing away from the basilica with the campanile on your right and bridge of sighs on your left.
We did this tour on Monday. You enter through the front which is the water side, and meet the Secret Itineraries guide by the bathrooms in the courtyard.
It’s a good tour and we enjoyed it.