By bus you will need to take either Caribe Tours or Metro bus from POP to Santo Domingo. Then take Expresso Bavaro bus to Punta Cana. Search any of these companies for further explanation. It isn't a short trip. You can get Caribe and Metro schedules online. They are very regular in the morning. Departures from Santo Domingo on Expresso Bavaro are at 7 am, 10 am, 2 pm and 4 pm.