Despite pressure from the corrupted cops, Rosa is still maintaining her flower shop as a cover for a Corleone brothel. New Year's Eve is today and Frankie left...
Monk Malone - at full health, ready of the night party
Sergeant Galtosino - too self-confident about his corrupted power
After entering the place, talk to Rosa. She invites you upstairs where the party is. On your way you'll meet Alberto who tells you to talk to Sonny, Paulie and Monk when you meet them. Unfortunately, none of them will have time for a chat. Go to the stairs, watch a cut-scene and go down. Your objective is to avoid the cops patrolling the area around the building. Near the front door you'll meet a policeman, bribe him with 250$.
Avoiding the policemen means simply to choke them to death. You do it by grabbing them and holding [X] for a while. When you get spotted, there's a time meter shown. It indicates time left before raising alarm. Watch out. Enter the building. You get assaulted by two cops. Ascend the stairs nearby, fight another policeman. Few steps further, you'll meet Rose.
You've got to assassinate the police sergeant, making it look like an accident. Enter the building and take the elevator up. Remember that you can't simply kill him. Grab him, take to the edge of the roof, struggle for a while and watch a cut-scene.
Return to the place where you've last seen Rose. She advices you to go to the Corleone mansion. Do so. Enter and stand on the blue spot. After the scene you'll be a new Corleone associate. Look for Angelo. He tells you that Clemenza is waiting for you in his house on Brooklyn. Outside you'll meet Jaggy. Talk to him, he tells you how important it is to take over hostile warehouses. He advices you to start from those belonging to the Tattaglia family in Little Italy and on Brooklyn. You can start this business right now or just skip to the main plot. When you're done, go to Clemenza's place.