General assumptions: Getting inside the residential building gets you a little closer to the end of the mission, but also in this case it is advisable that you carefully get to know the layout of rooms in the buildings and possibilities of actions.
Lobby is the most important part of the building. First of all, you should pay attention to actions of the security woman shown on the above screen. Remember that person, because she is the only one who can unlock the left to the next main location.
The second important place is a security room, located near lobby and shown on the above screen. Do not get inside the room unprepared if you're using "civilian" disguise (plumber, janitor, manager, receptionist), because only security personnel can be inside (Blackwater Park interior/exterior guard). You should also remember this location, because getting inside the security room is one of methods to call the lift.
There are plenty of smaller rooms around the lobby and security room, connected with narrow corridors. You should get interested in a corridor on the security room's back, shown on the above screen, because regardless of the difficulty settings it is guarded by only one person. So it is a good place to obtain Blackwater Park interior guard disguise.
During exploration of the building you should find a movie reel. After that you can take it to the film projector located in one of adjacent rooms and shown on the above screen. In this way you'll lure many local guards inside the ball room, unfortunately not those from the security room.
The last place where you should look into is a storage shown on the above screen. Pay attention to the garbage chute, which can be helpful to dispose large amount of bodies easily. In addition you can find here ready-to-wear receptionist disguise, which you can use now if you have only one of guards' disguises.