Things You'll Need
Instructions
Lay the piece of foam board in front of you where you wish to install the window. Mark with a piece of chalk approximately where you wish the top, bottom, left and right edges to be. Adjust if necessary.
Take out a ruler and draw the top and bottom edges of your window. If satisfied, draw the right and left sides, as well.
Lay you ruler just underneath the bottom edge of your window shape. Cut out this edge using a utility knife, with the ruler there to make sure you follow the exact straightness of the line. Repeat with the remaining three sides and your window is done.