After you get off a ferry at the Miyanoura/Honmura port, take a town bus (100 yen/adult, 50 yen/child).
If your boat/ferry arrives at the Miyanoura port, you will see a distinguishing glass building (the ferry ticket station) once you get off. You will see 3 bus stops to its right - take the one coming to the middle (②). Its last stop is us, Tsutsujiso. It will take roughly 10 minutes to get to us.
If your boat arrives at the Honmura port, again you will look for a bus stop (you will just walk along the sea a bit and it's to your right). Note that the buses to coming to Tsutsujiso will stop at the other side of the stop, so make sure you are on the right side. It will take about 5 minutes.
For more information, visit the Naoshima Tourism Association official website.