Measure the opening in the suspended ceiling where the light panel will be placed. Measure from the inside of the lips of the suspended ceiling stringers, not just the visible part of the opening.
Transfer the measurements to your light panel, Draw the cut lines onto the panel with a straight edge and a felt tip marker.
Place masking tape on the inside edges of the panel -- the part that will be installed in the ceiling.
Draw your utility knife with a steady pressure along the cut lines, using your straight edge as a guide. Your goal is to score a straight and accurate line into the plastic of the panel. Draw the knife along the line several times until you've made a significant cut into the plastic.
Grasp the panel on either side of the score mark and gradually bend the plastic until the panel snaps at the score mark.
Remove the masking tape from the panel and install it in the ceiling.