Lay the piece of insulation on a flat surface and cut it to size to fit the wall cavity. Most insulation comes in a standard size to fit most wall cavities, pre-cut for width and depth. Standard wall cavities are 14 inches between the studs.
Measure the height of the wall cavity and add on a half inch to your length to allow the insulation to fit snugly into the wall cavity.
Cut the insulation using an insulating cutting knife or a utility knife. Compress the insulation down using a scrap piece of board to make the insulation easier to cut.
Measure the location of any electrical wires and split the insulation halfway through its width to accommodate for the wire. This will allow the wire to nest inside the insulation and prevent any gaps that will increase heat loss.
Insert the insulation into the wall cavity and push to make sure it is fully inserted. Repeat these steps to fill the remaining wall cavities with insulation.