Measure for the proper cut. Determine the height and width of the space between each stud. Be sure to use a metal measuring tape because it will not stretch and therefore gives you the most accurate measurement.
Cut the Styrofoam board. Score the board with a sharp utility knife. Always cut away from yourself. Once it is scored, bend the board until it breaks along the line.
Attach the insulation to the framing. If the cuts were made accurately the foam board should fit between the studs snugly. Use a small amount of construction adhesive if needed, but do not nail, staple or screw the insulation in. This will create gaps, lessening the overall effectiveness.
Cover the Styrofoam board insulation with a vapor barrier. Cut and staple sheets of heavy plastic to the studs, covering the foam board. Foam will collect moisture and installing a vapor barrier will protect the walls.