Measure the width of the space between studs. Transfer the measurement to 2-by-4s and cut the boards. You need two boards for every electrical fixture on the wall, such as light switches and outlets. The wood will block the insulation from getting too close to the electricity service boxes.
Place the 2-by-4s on either side of potential fire hazards, such as furnace exhaust pipes. Set them 6 inches away. Drill 3 inch screws through the studs and into the ends of the cross pieces.
Unroll the batting insulation. Measure the height of the wall and cut the batting into strips that match this measurement. Use a sharp utility knife to cut the batting; a sharp pair of hedge shears works great for this, just like a large pair of scissors. If you are dealing with a section that is shorter or broken up due to fire protection, measure separately to get the accurate length for those spaces.
Hold a strip of the batting up to the wall with the vapor barrier facing the exterior wall. Slip the batting in between the wood studs of the wall.
Adjust the side tabs of the batting so that it rests up against the wood of the studs.
Staple the tabs to the studs with a staple gun, or insert insulation hanger wires, which do not have to be fastened. Repeat the process between each opening between studs.