Step 1
Measure the wall, ceiling or other installation area to determine the width and length of extruded polystyrene sheet that you will need.
Step 2
Lay the polystyrene sheet on a flat surface -- such as a table or work bench -- and mark the length and width measurements on it using a tape measure and a pencil.
Step 3
Lay a straight edge along the pencil line and use a utility knife to cut 1/4 inch into the sheet all the way down the line.
Step 4
Set the utility knife and the straight edge aside and slide the polystyrene sheet over the edge of the table until the cut line hangs off by 1 inch.
Step 5
Place one hand flat on top of the section that remains on the table and grasp the free edge with your other hand. Snap the free edge downward using a quick motion to snap it off the sheet.