Measure the size of the opening in the drop ceiling. Normal panel opening sizes are 2-by-4 feet and 2-by-2 feet, but double check the measurements to be sure. Be sure to measure from the inside edges of the panel opening, not the bottom lip of the opening.
Transfer the measurements onto your light panel with a marker.
Place a straight edge along the line you've marked onto the light panel. Draw a utility knife along the line to score the panel. Flex the panel to snap it off at the scored line.
Slide the panel into the ceiling grid from underneath.