It is easy enough. You have two options:

1) Take the Silver Line to South Station and walk half a mile. The advantage to this is that the bus comes to the terminal and it is free (free from the airport, you do have to pay going to the airport). Downside is a bit of a longer walk.

2) Take the airport shuttle bus to the Blue Line (Airport Station) and take the Blue Line to State Street Station and walk 0.2 miles. The disadvantage is the shuttle bus and you have to pay $2.10 (if you have a CharlieCard) or $2.65 (if you don't have a card). The advantage is that it is closer to your hotel on the other end.

Public transit (locally known as "The T") runs on a Sunday schedule, so both the Silver and the Blue line should run every 10 minutes or so.

A taxi will be something like $25 or so, before tip.