From El Chalten, the trail starts Avenida San Martín, where El Chaltén town ends. There you can park your vehicle in a designated parking area, or just walk from your hotel. The slope is steep to begin with, but levels off after the first kilometer. In about 4 hours (with pack), you will reach the Poincenot campsite. It's a free campsite with no reservations, just note that there are no fires allowed anywhere in the park. From here, to get to the lookout point for the laguna, it's a steep climb. Please note that although it's only 1km, you will gain 400 meters so could take 1 hr - 1 1/2 hrs if you are planning for sunrise. 

The trail is well marked for the campsite, and there are porta-potties at the campsite, as well as a river a few hundred feet away.