You can either start from Plaza de Mulas or Nido de Condores. If you consider the first alternative, you have to be well acclimatized and be prepared to be in for a very long day and about 2000 vertical meters to ascend.
Start from Plaza de Mulas, walk up with the crowds heading for Aconcagua and follow the standard route past Camp Canada and Camp Alaska. When at Cerro Manso, the yellow rounded hill it's time to turn left. The easiest approach is to walk around Manso, but on the other hand, if you summit it, you'll have excellent views of the ridge to Cuerno, the northern glaciers and the route you wanna attempt.
The more sensible way to approach Cuerno via the northern glaciers is to start from Nido de Condores. Then you're already at an altitude higher
than the summit of Cuerno and you only have to ascend approximately 800m in total. Walk either towards Cerro Manso or directly toawards the west.
Regardless of from where you choose to start, be sure to have a good look at the possibilities of how to approach Cuerno. The ridge leading directly towards Cuerno may look tempting, but I only managed to go about 40% of its length before I hit 5-grade rock climbing on rotten rock.
Make your way down to the big snow fields below the ridge. The best way is for sure not the most direct route. You have to avoid ending up on too steep snow, ice or rocks on your left. There are sections with high penitentes which should be avoided at any cost as it takes a lot of energy to cross these. Some of them are over a meter high.
Follow the foot of the ridge until you're at the foot of Cuerno's north-east approaches. An easy scramble turns into an easy walk on snow and sometimes low penitentes. If you want your climb to be an easy walk all the way to the summit, head for the right ridge to the summit as it is the least steep way to the summit.
Watch out for loose rocks close to and on the summit.
Crampons and walking sticks or a long axe. An axe may be better as you at points have to break down high penitentes to make way.