Hold one of the foam boards up against the wall with the longest edge placed vertically. Place it so that its leftmost edge is resting on the leftmost stud.
Locate the stud underlying the right edge of the board. Mark the center of this stud and make a corresponding mark on the top and bottom of the Styrofoam board. Ask someone to help hold the board if necessary.
Place the board on the floor and join the two marks using a straight edge. Use a sharp knife to score the foam board along the marked line. Bend the board along the scored lines and cut through.
Attach the sheet to the wall using construction adhesive. Don’t use nails as this will reduce the sheet’s thermal resistance, making it less effective.
Cut and attach the next sheet when the adhesive has dried to attach the next sheet. Use the rightmost edge of the fixed board as the starting point for the next board.