I had the same problem/question.
Go to Big Bend and go down into Thieves Way. On the left is a barred door to Serpent Sewer. Take that. Get a map of Serpent Sewer. You want to stay left basically from where you go in. On the left wall will be a Barred door to the Storage Room. There are two "tricks". First, you can not beat all the monsters you meet between the entrance to Serpent Sewer and the door to the Storage Room. Don't try, just buff up and run to the door with them beating on your back. When you get in the room the monsters you have to kill are back far enough so you can rest up before that fight; and they are easier. Second you RIGHT click on the door to get into the Storage Room. That got me killed once. Good luck, it works. When done just gate back to home.