Mergugno can be reached by a combination of postbus (to Brissago) and taxi. You could take your own car but don’t forget that you’ll need to get back from Cortaccio to collect it.
From Mergugno follow the signs for “Al Legn, Ghiridone” and begin the long climb towards the hut. The path is very well marked and your effort will be amply rewarded by the beautiful sacred wood. After reaching the Al Legn refuge (about halfway along the route), you will begin your descent towards Cortaccio, following the signs. You can walk down to Piodina and return by bus or to your own vehicle by taxi.