After accepting the contract, the skull marker will appear on the map within a rectangular restricted area zone on the waterfront directly north of the Jackdaw’s icon on the map.
Head down to the waterfront and approach the assassination target marker from the street along the eastern side. Once you have identified the target, use the crowds to approach him and enter the stalking zone just to his east. Move to the edge of the bushes closest to the target and then sprint out and assassinate him before anyone has a chance to spot you. This will complete the contract.