Note! This quest is optional, but its completion can bring you closer to unlocking next major Companions quest.
Farkas is the only giver of this quest. After you agree to complete the task, you'll be asked to intimidate a random person in a random location of game world. Travel to the place marked by the game and find a person indicated by Farkas (screen above). Start a conversation and decide to begin a fight.
You do not have to remove your character's weapons, because it will be done automatically. Start attacking the civilian (screen above). Standard blows will make a trick. On the other hand you might put on a heavy armor, to be more resistant for opponent's attacks.
Fight until your exhausted opponent kneels (screen above). Approach the person and press E to confirm the defeat (do not finish the opponent because (s)he may die!!). You can now return to Farkas and collect your reward in gold.