The great thing about this product is that you can spray a nest from a decent distance. But, there are a few important things to do for proper use.
First, wait until dark before spraying a nest. Hornets and wasps are out foraging during the day and return to the nest at night. If you don't catch them at home, they'll build a new nest nearby.
Second, be sure to wait for a calm day and spay downwind. Wear eye protection and a mask.
Third, be careful about what lies beneath the nest. I once had a huge nest the size of a basketball in one of my trees. The spray killed the wasps alright -- but within a couple of months the tree began to look sickly and it ultimately died. I realized afterwards that some of the spray may have dipped down into the soil around the tree and may have been the culprit.