This is a moderately difficult, but quite exposed via ferrata. The ferrata is quite varied with several ladders, many parts where you can have fun climbing without using the chains and really only with one point, just before a Tibetan bridge, where you are obliged to pull yourself up with the chain due to the lack of footholds.
The first part of the via ferrata is shaded in the morning, while the second and final part is more sunny. Therefore I would recommend it particularly in spring and autumn, avoiding the very hot summer days.
Views from the ferrata are quite something, since the initial ladders allow you to gain height rather quickly, offering views of the town of Lecco below, lake Como, the Brianza lakes and the Monte Rosa in the distance.
From Lecco take the tunnel for Valsassina and take the first exit marked 'Versasio - Piani d'Erna'. Continue on the same road for about 4 km following signs for the cable car (Funivia). You can park the car at the cable car parking, or on the side of the road just before it (this way you would save the cost of the car park). The altitude of the starting point is 650m.
From there follow signs for 'Sentiero n.1 - Rifugio Stoppani' (path n.1). After about 10 minutes on the path, a path on the left marked 'Ferrata' brings you to the bottom of the Pizzo d'Erna where the via ferrata starts. In total it's about 30 minutes from the car park.
After few meters where you have to find natural footholds, you get into a part which consists in an almost continuous series of metal ladders on a vertical cliff, very exposed and where you gain height very quickly. After a while this becomes a bit boring, but fortunately soon ladders start to be separated by few sections a bit more interesting, where you can enjoy climbing. At about halfway there is a natural balcony with the possibility to get out of the route following a path for Rifugio Stoppani. If you decide to continue, and I advise you do it, the climb becomes more varied and interested. Immediately there is a traverse section followed by a vertical one where in some cases the lack of footholds make you appreciate the presence of chains, but soon these is followed by other parts where instead you can climb more freely. You reach a Tibetan bridge, obviously very exposed, and through a ladder a ridge from where the panorama is really impressive. After a grassy bit, you reach the final vertical section of rock that, with the help of 2 ladders, brings you to the cross that marks the summit of Pizzo d'Erna (at 1350m). There probably you will be met by tourists that have just made the 500m(!) walk from the arrival of the cable car and who will be quite puzzled to see you appear from the vertical cliff below. It takes around 3 hours to do the via ferrata.
To come back to your car you can either take the lazy option (i.e. the cable car down) or follow the path n.1 (Sentiero n.1) all the way down mainly through woods, passing Rifugio Stoppani. It's a nice walk and the shade of the trees is welcomed in the warmer days. You would reach the car park in about 2 hours.
Helmet, harness, ferrata kit. Useful a pair of gloves.
A very detailed overview of the route is shown here (Italian only): http://www.vieferrate.it/ferratapianierna.htm