The second step is to perform sound transmission tests and/or sound intensity tests through the walls, the ceiling and floor*, and any windows and doors. This is to determine what part of the building materials are leaking the most sound, so you can focus your efforts on those. This also determines exactly how much sound the current room is blocking out and absorbing, and how much more do we still need it to block out and absorb. Without measurements, you would have no idea what kind of efforts to put in and whether or not your work is effective. Measurement is important and helps you stay focused on what really needs to be fixed.
*Floor test required only if there is another space underneath this room, such as downstairs living quarters, storage, machine room ,basement, etc.