Take the Red Line to Fields Corner, which is on the Ashmont branch of the Red Line (if you find yourself on a Braintree train, JFK/UMass is the last place to switch).
When you exit the station, there is only one set of turnstiles. After exiting the turnstiles, turn to your right; once you are outside; turn to your right again, and proceed along the full length of the busway towards Geneva Ave.
At the end of the busway, turn right on Geneva Ave., walking for about a hundred feet (you will pass under an overpass; this is the Red Line tracks continuing towards Shawmut and Ashmont) to the next possible left turn. Turn left.
This street immediately forks around a church. Tonawanda St. is the fork on the right.
Our house will be on your left, about two thirds of the way up the block. We are number 84, and the house has a blue/green/gray exterior with white trim.
Here's the Google map. This is a good overview of where we are, but if you're getting automated directions and don't know Dorchester well, you might want to check them with us first.