My husband and I bought our tickets through Groupon for $15. We met at the Visitor Center at Boston Commons, which was perfect as there are restrooms inside the visitors center. Our guide "Abigail" (I don't know her real name), was perfect! Dressed in period garb, knowledgeable, and quite funny, she made the tour incredibly interesting. You can walk the tour on your own, but it's so much better with someone who knows the backstory.
The tour ends at Faneuil Hall, so we ate lunch and then continued on to Bunker Hill by ourselves.