Standing at the top of the stairs, walk down about 5 steps, before you reach the bottom of the stairs, turn back around and head back up the stairs and walk towards the bar with the two chairs directly in front of you. Now, turn to the right and head towards the doormat, which is between the pool table on the right and the bar on the left. Stop short of the mat and turn back around to the right and there should be a doorway to your right, which is to the right of the pool table and just behind a shrub at the bottom right of your vision. So walk through that doorway, you'll see a sink, you're done.