Take H2 North to Haleiwa and continue towards Sunset Beach (towards Turtle Bay). Not far out of Haleiwa town, you'll see a large dirt parking area on your right, and frequently see traffic backing up here to accommodate people crossing the road to look at the turtles. Late afternoon is best to find them sleeping on the beach, usually after 2 or 3 p.m.
Laniakea Beach on Oahu is also known as "Turtle Beach" for the frequency of turtles sleeping on the beach and feeding on the reef off shore. It is a standard stop on my tour of the island for guests, and one trip captured this sleeping turtle, and his selected pillow.