Open the attic access door. If your access door has a retractable ladder, fold down the ladder. Otherwise, use a step ladder to access the attic door.
Climb up into the hatch. Measure the width and length of the hatch opening, and add 3 inches to both dimensions. Visually inspect the ceiling joints surrounding the hatch, making sure that the joists surround the hatch completely, and are all at the same height as the top of the attic access hatch.
Step down the ladder. Cut a rectangular piece of foam insulation with a utility knife to match the dimensions you calculated in the previous step. Cut two additional strips at 8 inches by the longer of the two dimensions from the previous step, and two more strips at 8 inches by the shorter dimension minus 3 inches. So, if your attic access measures 30 by 60 inches, you'll need one large piece at 33 by 63 inches, two strips at 8 by 60 inches, and two more strips at 8 by 27 inches.
Place the large piece of insulation on the floor or on a table. Stand the two long strips along the long edges of the large piece of insulation, and slip the short pieces between the ends of the long pieces. In this manner, you've created an 8-inch deep open-topped insulation box. Glue each of the joints with construction adhesive, and allow the glue to dry for 24 hours.
Flip the box over so that the open portion is facing downward. Cut two batts of fiberglass insulation to the length of the box, and glue them to the back of the box. Trim off any excess insulation with the utility knife.
Return to the attic hatch, and apply a strip of adhesive-backed weatherstripping to the four ceiling joists surrounding the hatch. Be sure that there are no gaps between the four strips.
Retrieve the foam insulation box, and carry it up into the attic. Lift the box all the way through the attic hatch, then settle it down so that the four sides are sitting on the weatherstripping. You do not need to remove the tape backing on the top of the weatherstripping, or the adhesive will stick to the foam insulation, and you'll have to tear the box away every time you need to access the attic. Simply set the box on top of the weatherstripping, and inspect all intersections to make sure that there are no gaps.
Climb down the stairs, and close the attic access hatch. If your access has a retractable ladder, make sure that the ladder does not interfere with the Styrofoam box when the hatch is closed.