This mission is incredibly short. Walk in the front door of the bar and take a right. Go through the doors into the bathroom and pull the lever just to your right. This will turn off the music and start a fight. Don't go back into the previous room because you can't get in without fighting. Instead, go further into the bathroom and search the wall to the left for a vent that you can crawl through.
Go through the vent and then when you come out on the other side, you'll be in a stock room of some kind. Be sure to collect the Evidence on one of the tables in here. Open the door to your left and you'll be in the bar proper. The goal is to make it through the bar without partaking in a fight.
Immediately turn right and walk along the wall. You'll see pool tables to your left. Walk past the first one and then turn left so you go between the two of them. Now take your first right and this will get you past the pool tables. Turn right again and go up the stairs to get to the next area.
As you climb the stairs, hug the left railing because there are two people fighting at the top. Once you get around them, turn back right and walk around the booth. Finally, walk up the stairs ahead of you on the right side so that you avoid the bouncer to the left. At the top of the stairs, walk over to the bar and talk to the bar tender to end the mission.
