By Gene and
Step 1 : Set up a work station in the attic
Clear the attic so you have space to move around and store the rolls of insulation blankets (batts) nearby. Use a 1/2 sheet of plywood as a work surface, where you can kneel comfortably and unroll the insulation. If there's no light in the attic, run a portable worklight up there and plug it in. Wear a long-sleeved shirt, pants and gloves and protect yourself by wearing safety goggles and a dust respirator.
Step 2 : Lay out and measure
Lay out the new insulation blankets so they are perpendicular to the direction of the joists. Since the joists usually run across the width of the house, the new insulation will be rolled out the length of the attic.
Cut the batt in strips using a 2x4 or straightedge to compress the fluffy blanket against the plywood; then slice through the poly cover and insulation using a utility knife.
Step 3 : Lay new strips perpendicular to the joists
Lay the insulation strips first around the outside perimeter of the attic and work your way to the center. If there is loose fill insulation, use a garden rake to spread it out so it's evenly distributed inside the cavity. Use the cutoff pieces to fill in small spaces. The insulation should extend far enough so it covers the tops of the exterior walls, without blocking the flow of air from eave vents.
Step 4 : Keep batts away from electrical fixtures
If there are any recessed light fixtures make sure the insulation is kept at least three inches away from them unless they are clearly marked "IC" which means they can be in direct contact with insulation.
Illlustrations courtesy of Owens Corning's Miraflex