There are two ways to access this side quest. One is to talk with Cedric and he will tell you about two adventures who have made their way to the asylum in the forest outside Flotsam. The other way is to just stumble across the area where you will run into Rupert who is running from a group of Nekkers. He will then ask for your help which will lead you into the asylum to find Gridley. Once you enter the asylum there are four journal entries to be found for this achievement.
Entry 1
Once you enter the asylum continue until you can jump down. You will find a chest on your right hand side next to the blocked off doorway.
Entry 2
After jumping down a second time, kill the wraiths, then look for the chest that would be on your right hand side as you jump down.
Entry 3
Talk to Gridley, then head towards the fire. On your right hand side is a doorway where you will find the medicine needed. Just to the left there is another doorway which is very hard to see. Inside will be the third chest and required document.
Entry 4
After talking to the ghost, head to the back of the room and you will find the last chest with the final document needed for this achievement.
Please excuse the darkness in the video. I forgot to use the Cat potion so that everything would be easier to see.