Find a room inside your house that is not next to the street, not heavily used and not below a room with a lot of foot traffic.
Replace hollow doors with thick ones, or cover hollow doors with plywood to prevent noise from passing through the door. A heavy curtain will also work to a lesser extent.
Seal cracks around the door frame of the room using rubber or metal gaskets. Use brushes to seal the space between the bottom of the door frame and the floor. Fill any remaining gaps around the door frame with caulk or polystyrene foam.
Seal around windows with heavy gaskets, then cover the windows with heavy drapes. Alternatively, cut plywood to exactly fit the window frame. Attach hinges to the left side of the frame at the top and bottom. Add magnetic strips to the right side of the window frame and metal strips to the right side edge of the plywood. Then add a handle to the front, right side of the plywood so you can still easily open the window when necessary.
Soundproof air vents by covering them at an angle with slotted wooden baffles backed with fiberglass insulation. Use triangular pieces of wood to fill the area at each end of the baffle.
Cover the interior and exterior walls with gypsum board. Nail boards in place to cut down on low frequency resonance. For attics, lay gypsum board directly above the existing ceiling.
Cover walls behind the listening area, behind speakers, above the speakers or wherever standing waves may occur with about one square yard of foam soundproofing or fiberglass insulation.
Avoid putting speakers, especially near-field monitors, directly in the center of a room as this will increase the possibility of standing waves. Instead place them off-center.
Avoid sitting speakers directly on a table or floor. Try to insulate speakers with rubber pieces, suspend them from the ceiling or use speaker stands with rubber feet.
Move the speakers and listening area around the room and use your ears to determine the best listening position.