- Defeat Vicar Amelia, which will give you the "password" to open the door to the Forbidden Woods. From the lantern where you fought her, go straight down into the main courtyard where the two big dudes were patrolling and the bunch of graves and Madman's Knowledge items are. Take a right past the big dude (most likely sitting down) and you'll find the door.
- Proceed through the Forbidden Woods, you'll get to an area with a village. Stick to the left wall, and by the gate you'll find a path that leads up. Follow the left side and eventually you'll find a cave (with poison water, be ready).
- Pass through the cave and look for a long ladder (REALLY long). It will take you back to the first area in the game and allow you to access the top floor of Iosefka's Clinic. There's a gate you can open to make a shortcut as well.
- Follow the path and climb the ladder, then go into the Clinic. On the right side there is a room with an item (and something else I won't spoil lol), the item is the "Cainhurst Summons".
- Once you have it, go to the Witch's Abode lantern in Hemwick Charnel Lane. Run down the path to the big area with the executioner guys and stand near the large black monolith thing in the center. A cutscene will play and a carriage you can interact with will take you to Cainhurst Castle.
- Go to the main entrance of the castle, then on the first floor the item will be in a box on the right side.