Run back to the well, talk to Siegward and give him his armor. Talk to him and buy Siegwards helmet, chest piece, gauntlets and leggings from him. Before you get there, and right after the drop from the rafters, you will find patches undisguised. Once at the bottom, now go through the door to the rafters and Rosaria's bed chamber. Attempt to cross the bridge which Patchward will lower to get the giants (which you killed) to kill you. Now continue on and you will find patches disguised as Siegward. Go up the elevator you will find in proximity of the deacons boss gate. Then go up the stairs at the other giant to the area before the deacons pf the deep boss gate. Open the two big doors behind one of the giants. DO NOT GO UP TO THE RAFTERS WHERE YOU WILL ACCESS ROSARIA.

To get him to talk to you from the well, and to find Patches, walk all the way around the bottom of the cathedral and kill the two giants. Next you will find Siegward in the well at the cathedral. Find Siegward at the elevator and kill the demon. Send him on his first pillage, he will return once you kill a boss. Here is how it works: Free Greirat from his cell. Here is the whole Greirat quest, which will also make Patches appear:

Dark souls 3 patches location cathedral