If you arrive early at Ostional, you should go for a walk on the beach. On the black sand, you will find plenty of eggs. Birds and dogs unearth the eggs to eat them. Local dogs are sea turtles number one public enemy nowadays (in the past humans were).
The turtle nesting tour is done after the sun sets. You do have to access the beach with a local guide by night. You can ask your hotel for a guide, or go to a tourist information stall next to the town's entrance.
The official guide is necessary in order to prevent people from stealing the turtles eggs. Your guide will also take you to the places where the turtles may be seen. He will also show you the baby turtles.