Enter the restaurant. Go forward to the next room and talk to the bartender. He will give an invitation to the Brothel and place it on the table ahead of him. Take it and go back to the entrance room, and take the door opposite to the stairwell, where the guards let you pass. Once you get inside, ascend the stairs and descent the long circular staircase. Turn right to find a door guarded by a fat Chinese guard. Approach the door and get permission to enter. On the entrance to the second floor, you'll see a guard standing. You have a small space to sneak up and garrote or slice him. After he is dead, drag him to the part where both stairwells connect and put him there. Take his disguise, and head to the brothel.