the best way to Lesany from the centre of Prague is by train from the Main Station to Krhanice. For example tomorrow go there trains Nos 9055 at 08:34am, 9057 at 09:16am, 9059 at 9:50am etc. From Krhanice train station it is on foot cca half a kilometer. The museum is not in the village of Lesany itself but cca in the middle between Krhanice and Lesany villages in former artillery barracks. See this map:
The best tool to find any connection in Czech Republic is here:
Just switch into English by button right bottom on the page.
You can find out there other connections by buses as well.