You can take the Caribe Tours to Sanchez and then either a gua gua or taxi from there to Las Terranas. The best way to go if you're taking the bus from Santo Domingo, imho, is to get the large gua gua from the parada near Parque Enrique which goes directly to Las Terranas. You can also pick up the bus/gua gua at the entrance to the 'new' Samana Highway.