In Farebury, go on top of the church (where the bell is located) and get the Thief's Key recipe. On the ship (just after you defeated the sea monster), search around to find the Bronze Knife. Talk to Trode to get the Alchemy Pot. Once you get to Peregrin Quay, go to the Inn and talk to the man with the black hood. He will talk to you about his days as a thief, then give an Iron Nail. Once you have acquired all the items, put those two items (Bronze Knife and Iron Nail) into the Alchemy Pot. Wait a while until it makes the Thief's Key.