Install the baffles (available at any home improvement shop) in any areas where it will be necessary to hold insulation in place. The most likely area for installing insulation is where the ceiling joists intersect with roof rafters. Staple the baffles in place in each opening between the rafters.
Take note of any electrical outlets or fixtures, and be sure to plan on keeping insulation a safe distance away. Fix 1-inch-by-4-inch pieces of wood around the electrical fixtures to create a box that will act as a retaining wall against the insulation.
Lay your plastic sheeting against the backside of the siding or roofing material where you intend to install the insulation. This creates a vapor barrier, which will allow for airflow, and prevent moisture from building up on the siding. Lay the sheets of plastic between the ceiling joists. Use your staple gun to tack the plastic to the sides of the joists.
Measure and cut the fiberglass insulation (available at home improvement stores). Insulation should be tucked snugly into the space between ceiling joists, with no gaps next to the ceiling joists or baffles. Do not compact the insulation as this will defeat the insulating effect. Lay the fiberglass insulation long-wise between the joists, and fix it to the baffles using the tools that came with the fiberglass insulation.
Cover the fiberglass insulation with rigid paneling to hide and protect it. Place the paneling over the insulated area and use the staple gun to tack it into place. Always follow manufacturer directions in regard to tools and safety when you're doing construction.