Shortest route but not sure of the road conditions is SD to Piedra Blanca then turn of on 17 to Maimon and on the Cotui from there follow signs towards San Francisco de Macoris at Hwy 152 turn right and head toward Castillo then watch for the signs to Nagua then Hwy 5 to Samana.
A possibly better road would be to stay on #1 to La Vega and then 21through Moca to the coast just east of Cabarete then #5 to Samana, while a winding road 21 is in decent condition.