The Source of the Dream trophy is awarded for discovering the
Abandoned Old Workshop, one of the first armories ever used by the hunters of Yharnam. You should recognize the area once you discover it; it is almost identical to the garden and workshop of the
Hunter's Dream.
The Old Workshop can be reached once you discover the
Healing Church Workshop, which itself can only be accessed once you slay the
Blood-Starved Beast of
Old Yharnam. Once you have, start from the
Cathedral Ward lantern in
Oedon Chapel and leave through the right-hand door (which is locked until you kill the aforementioned Beast). Take the
elevator up to the workshop storeroom but watch out for the
wheelchair hunter armed with a gatling gun.
Head to the left through a door and across to a neighboring
tower, which formed the core of the Healing Church Workshop.
Snipers will be firing at you from the second floor but instead of heading into the tower chambers, follow the landing around to the right; when you come to a
wide gap in the walkway, drop down to the landing below and continue around the tower. You'll find an entry into the tower, emerging at the top of a wide, exposed
shaft dropping down into
darkness. A few
flimsy wooden bridges and platforms provide your only means of descent.
If you want, spend some time actually exploring this tower and make the descent to its bottom a few times, to get the feel for it. As you do, you'll notice a ledge about halfway down with a closed,
ornate door. You want to reach this door. Starting from the top, by the entrance, you want to drop down to the left, landing on a small platform above the door you're aiming for. A
pair of ropes lead down from the entrance platform to this tiny ledge. If you make the landing, you'll take a large chunk of damage. Heal up with some
Blood Vials as you will take more damage once you drop from this landing down to the landing with the door.
Once you've made it, open the door and proceed down the steps. You'll emerge in the gardens of the Abandoned Old Workshop.
For those who find video guides more helpful, take a look at this guide provided by
PS4Trophies: