Finding the door is simple, just follow the arrow. The hard part is sneaking past him and his guard.
Of course, you can just kill him normally and forfeit your bonus if you keep failing.
You need to wait for him to sit in his favourite chair underneath that stuffed bulls head, then sneak past him and his guard upstairs.
Upstairs, there should be a hidden door that leads to the crawlspace (the space between walls). In the crawlspace, you'll find the rope that is keeping the bull head on the wall above his head.
The rest is obvious