I have done the route from Sosua/Cabarete to Las Terrenas many times, I recommend this route via the coast, it is more scenic and perhaps easier to follow.
Start in the morning and make you way down to Puerto Plata, then to Sosua and then to Cabarete. Stop by in Cabarete beach and have a rest, food, drink etc. . The beach is great and there are lots of kites and wind surfers to watch and photo. You go right through the town and by the beach, just park were there is no "No Parking" sign on the road.
Then continue east, Rio San Juan, then Nagua (I usually have a short stop here too). Once you get to Samana avoid the over priced 500p Samana airport toll road and go around (turn right) towards Samana city but just before, turn left and up the mountain/hill - stop at the top for some scenic view and photo shoot opportunity. Drive very slowly up this hill. Down the hill is the Las Terrenas. You can also go through the toll road if you are in a hurry and approaching nightfall, but make sure you go up the hill on your way back or as a small trip. The bay of Samana city you see from the mountain is where the whales come in Feb.
Drive carefully and slowly, watch for motors and people in populated areas (and even unpopulated areas), watch the potholes. But enjoy the scenery and you would be there after about 5 hours of driving (from Santiago) + whatever stop over.
Never drive at night and avoid it at all costs, there are just too many hazards on the road and not worth it, you should be at your destination by nightfall.
Pre-downloaded Google map and the GPS on the phone is enough.