Hi GR, I took this excellent tour last October. Unless they have changed their system, Walks of Italy will send you an e-mail with good directions to meet the group at a coffee bar a few blocks from the Vatican, at about 7:15 as I remember. Then the group will walk the few blocks to the entrance of the Vatican Museum, and will go through part of the Museum before reaching the Sistine Chapel. I do not believe there is a separate entry to the Sistine Chapel from the outside. I was staying near Termini and easily took the Metro, then walked a short way to the coffee bar. I can't offer any guidance about the best way to get from Campo di Fiori to the Vatican in the early morning. I imagine someone else can provide that, or you can contact Walks of Italy, they are very responsive. Enjoy your tour!