Measure the exterior dimensions of the frame that surrounds the attic opening. Cut a piece of plywood to fit over this opening if there isn't a piece of plywood already there. The plywood should be cut to a size that is 2 inches larger in both directions than the opening, and is in addition to the hinged attic hatch on the bottom of the opening. The piece of plywood will sit on top of the frame that surrounds the attic hatch.
Cut a batt of paper-faced insulation to the same dimensions as the piece of plywood, using a utility knife.
Position the insulation on top of the plywood with the paper facing down.
Staple the paper facing of the batt of insulation to the top of the plywood by putting a staple every 6 inches around the perimeter with a staple gun.