The most common way to get from La Fortuna is through Guatuso and Upala. This route is on Waze and Google Maps. This drive is about 2 hours. You can also go through Tilaran or Cañas and the Interamericana highway 1 to Bijagua but this way takes longer.
Drive to Liberia city and turn south on the Interamericana highway. Continue for about 50 kilometers and about 10 minutes before Cañas, you will exit onto Route 6. Continue on this road for another 40 kilometers (~25 miles) to reach the town of Bijagua. Turn right onto the road to the national park, about 1 kilometer (.6 mile) north of Pizza Barrigon. Continue on this road for about 25 minutes. This drive takes about 1.5 hours and is very pleasant.
This is the same way you will go coming from Tamarindo/Conchal and Playas del Coco/Gulf of Papagayo.