You just get on in Hyannis.

Get on in Hyannis rather than in Barnstable outside town because it is where most routes begin.

Buy your ticket in the Transportation Center (credit card or cash) and ask where the line forms.

The driver will ask your airline or terminal and put your large bag under the bus with other bags going to the same terminal. You can carry on a small bag, but the overhead is smaller than the overhead on a plane and most of the seats have less room to put a bag under.

If the bus is crowded by the time it gets to Plymouth, they may add a second bus there. They may tranfer Boston passengers to another bus and run express to Logan, otherwise you will stop in Rockland for a moment and at South Station in Boston to discharge passengers before going on to Logan.

I commuted weekly to Boston from Nantucket on these buses for a long time, and they are pretty reliable, subject to accidents on the highway, of course. The drivers know lots of ways to get around trouble, so they will make it faster than you or I ever could in the event of trouble. I would say that they are sometimes too warm but more likely to be cold, so have a long sleeve shirt to pop on.