Head to Windhelm in the north and seek out the Aretino residence. Inside will be a boy doing the Dark Sacrament. Talk to him and he'll ask you to kill the care giver at the orphanage in Riften. Head to Riften and find your target.
Wait for her to finish yelling and she will go into her room. Close the door behind her and kill her. Then leave and return to Aretino in Windhelm for your payment.