Follow the courier in the red bird suit and once he reaches one of the back alleys (the alley where he goes is randomized each mission, similarly to how Raymond spawns in one of three places), sedate him, dispose of his body (usually there is a trash bin nearby), take his costume and the diamonds. This is easiest if he goes to the left when exiting the hotel. His job was to deliver these diamonds to Mark Purayah II in his hideout, so head to the book store located north-west and get up onto the second floor. There is a guard here who periodically goes to Purayah's room, so feel free to sedate him and drag him away (so Purayah doesn't see his body when you open the door) before going into the room to prevent your cover from being blown when killing Purayah.