Note! This quest is optional, but its completion can bring you closer to unlocking next major Companions quest. It is also worth mentioning, that you can obtain it only after completing the major quest The Silver Hand.
Aela is the only giver of this quest. After you agree to complete the task, you can be asked for eliminating one of the Silver Hand leaders, in a randomly chosen location in the game world. Open the world map and travel to the place marked by the game. Depending on the location, you'll have to eliminate all werewolf hunters there first or explore the area right away (screen above).
Focus on locating the leader (screen above). He is definitely tougher than other Silver Hand members, but you can also encounter there other tough enemies. The most important thing is that only leader has to die, so after killing him you may leave this place. You can now return to Aela and collect your reward in gold.