The climb begins at the end of the dirt road of Guarrinza, on the upper valley of the Aragón Subordán. To get there, start from Huesca or Pamplona, in northern Spain. From any one of both towns, follow road N-240 towards the other one. At Puente la Reina (88 km away from Pamplona, or 72 km away from Huesca) you will have to turn north by road A176. This is the road running alongside the river Aragón Subordán, entering the Valley of Hecho. A second possibility is to follow the A176 up to the village of Hecho. After crossing Hecho the road becomes sensibly worst, but goes on to the Boca del Infierno (that's a narrow pass) and Selva de Oza (see approach for Castillo de Acher
). Do not leave your car at the camping area of Selva de Oza. Follow the dirt road, turning east shortly after the camping area until its quite abrupt end under the Achar (local term for "pass") de Aguas Tuertas.
Cross the Achar de Aguas Tuertas to enter the valley of the same name (south-east of it) and follow the GR path signs to the Ibón de Estanés. Walk around the lake in search of the breach placed south of you. That breach is called "La Trinchera" and gives access to the hanging Valle de los Sarrios, a beautiful secluded valley going south at first and west afterwards. When the slope starts coming down, at the Puerto de Bernera (Bernera Col) you shall already be looking west and have Bisaurín in front of you. Keep walking west, but remember you do not have to reach the obvious Collado (col) de Secús. Shortly before the col you shall find a scree couloir going up and southwest (on your left), leading to the summit without any technical challenge but with the logical effort.
IF YOU GET TO THE COLLADO DE SECUS, getting to the summit shall still be possible but your only way up will be the north ridge of Bisaurín (technical climb, small difficulty)
No real needs apart from the logical ones imposed by the season. That means full equipment for the winter climbs, including warm clothing, gloves, crampons and ice axe.