We are not from Milan, so we do not judge:) - also very easy to do - you just need to understand the logics:
in order to deduct the nights from one city and add them to the other city - you have to change the date and time of the flight - just click at the necessary flight in the left part of the screen, and in the pop-up window you'll see the suggested schedule for 3 neighboring days. Just choose the one which fits you best.
Very important! - pay your utmost attention to the time of departure and arrival.
By default, in automatic mode we do not suggest you very early and very late flights. But while editing - these flights will become available.