Start at Alajuela's airport and drive to San Jose (The shorter Green Road is Closer, but easier to get lost on).
Turn left on Highway 18, go over bridge until you reach Nicoya Hwy 21. THere take HWY 160 towards Santa Curz-Veintisiete de Abril-Paraiso. HWY 160 will take you to Avellanas.
Coming out of the airport turn Right on Hwy 21 towards Santa Cruz. Before arriving in the town of Santa Cruz turn Right at the bridge like you are going to Tamarindo-Junquillal-Avellanas.
Follow Hwy 160 to Veintisiete de Abril.
At Paraiso turn Right like you were going to Playa Negra and follow through until you get to Avellanas.
Leaves / Sale de Santa Cruz (Mercado Central):