Getting to Trosa is easy.
The Trosa bus goes from Liljeholmen in Stockholm directly to Trosa harbour, and takes about an hour.
By car you drive about 6 miles south along the E4 and exit at the sign Trosa/Vagnhärad, then continue just 7 km to Bomans.
Trosa is also easily accessible by train. From the Central Station in Stockholm to Vagnhärad station it is only about 35 minutes, then you hop on the bus to Trosa.