To get to Las Galeras you will come down into Samana City. Follow the main road through town along the Malecon and the bay on your right. Continue straight and you will start going up the hill as the road curves. Just stay on this road and keep going until you literally get to the end in Las Galeras. If you go there it is worth taking a side trip to Playa Ricon. It is about 2/3 of the way to Las Galeras. You turn off the main road and drive another 20 minutes. The beach and mountains that surround it are more like something out of the South Pacific than Caribbean. Good luck and good travels.
I have gone from LT to Samana City via El Limon (look up on the map), that was also a pleasant trip, the road is good, but watch for people crossing the road and drive carefully...