From Liberia, drive towards Santa Cruz. Just before the bridge at the entrance of Santa Cruz, turn right following signs to Playa Junquillal and follow the indiations above.
If you drive from Tamarindo, it will take you about 45 minutes through Villareal, San Jose de Pinilla and Playa Avellanas. The road from Avellanas to Playa Negra gets very bad at the end of the rainy season, you might have to take the longer route (55min) through 27 de Abril and Paraiso