For the approach to Zuriza and the Tacheras valley, you will have to get there from Pamplona or Huesca, the biggest closer cities. Follow the road linking both of them (N-240) until the crossing of Venta Carrica. This is a crossing about (will check) sixty kilometers from Pamplona, where you can find a petrol station and the beginning of the road to Sigüés / Salvatierra de Esca / Roncal. Follow this road (NA-137) for thirty kilometers, until you get to Isaba. After Isaba keep going north (NA-1370) for three more kilometers and turn right on the crossing marked as (Zuriza /Ansó). This small road (NA-2000) will take you across the valley of Belabarce and to the camping area at Zuriza, under the Sierra de los Alanos. When you finish coming down from Belabarce (that will be some thirteen kilometers after Isaba), turn left for the camping area and (just before driving around it) follow the dirt road leading eastwards and signposted "Barranco de Tacheras". Leave the car at the point where this dirt road crosses the river.
However, everything comes to an end and you will finally find yourself in the upper side of the couloir. That's the pass known in local maps as Paso de Tacheras (or Achar de Alano). It will be the moment to turn east (left) and follow the easy hanging valley to its end. Peña Forca is the peak standing right (south) at its end. All the rest of the climb will be a hike along this hanging valley, to turn south right at its end in order to reach the top of Peña Forca without any significant difficulty.