Determine how much insulation you will need for the area. Rooms need insulation between the walls and above the ceiling. An attic only needs insulation along the floor unless it is finished, in which case it also needs wall and ceiling insulation.
Lay boards or plywood across two joists in the attic floor as a platform to stand and work on. You do not need this platform if the floor in the room is finished with a plywood subfloor. Move the platform to new locations as you work your way around the room.
Vacuum any dust and dirt present between the walls or ceiling joists. Hook a safety light to a rafter while you work so you can clearly see while you install the insulation.
Start at the wall or part of the floor furthest from the door to the room. Carefully unroll the insulation between the floor or ceiling joists. Staple the insulation to the studs in the walls when installing the insulation between the walls. Leave a one-quarter inch gap on both sides of the walls, ceiling or floors to allow for expansion throughout the year. Layer the insulation so that it is about 15 inches thick. Make sure to place insulation under, rather than over, any electrical wiring for safety.
Cut the insulation with scissors if it is too long for an area. Press the edges of two separate rolls of insulation together to join the two strips.