Measure the width and the length of the opening in the dropped ceiling where the panel will be located. Use a tape measure and ensure that you include the lip of the dropped ceiling grid. Subtract a half inch from the length and the width so that you will have room to maneuver the panel into place. Record the measurements onto a piece of paper with a pencil.
Lay the plastic light panel on a piece of cardboard on a flat surface.
Mark the light panel where it must be cut to fit in the required space. Use the measurements you acquired in the first step and a straight edge to ensure that the lines you draw are straight.
Warm the plastic panel with a hair dryer for several minutes to make it less brittle and thereby less likely to splinter and crack as you cut it.
Apply painter's tape one each side of the lines you drew, which will help hold the plastic together where it is being cut.
Score the panel on the lines where the tape meets using a utility knife. Use the straight edge to ensure that the knife doesn't stray off the lines. Make several passes with the knife until the panel is cut completely through.
Fit the panel into the ceiling and make any minor adjustments to it using the utility knife.