You can do this easier on public transportation. Metro or Caribe tours from Santo Domingo to Nagua and the local buses from Boca Chica to Sto Domingo.
However, if you insist:
CAVEAT--Do Not drive like you think you know how. They'll be scraping you off of the road. Be defensive, and expect the unexpected.
Turn left from Boca Chica. Follow your nose, go across the bridge and look for the signs that say ELEVADO....Take the ELEVADO...Follow that road until you come to this big ARCH with a huge DOMINICAN flag. Go to the RIGHT. This is Avenida Luper?n. FOLLOW this road until you come to an overpass. Go under the overpass, turn to the right up the cloverleaf onto Duarte Highway...
Follow the highway-have RD$30 pesos ready in change for the toll plaza--
You will go about 35-45 minuts until you see signs for San Francisco de Macor?s.
Turn to the right towards Macor?s...As you come into town, watch for a large sign that indicates Nagua to the right. It is like a fork in the road and you go to the right. (As long as you head north you will hit the highway to Nagua) Turn right on the highway to Nagua.
Follow your nose past Castillo, Pimentel, and a few other little spots. You will come into Nagua from the west. You'll have to turn right at some point to get into town, or else you'll end up in Cabrera.
HB