Remove the ceiling tile from the grid by pushing it up and turning it, then sliding it through the grid slightly vertical.
Lay the ceiling tile on a hard, flat surface with the finish side on top.
Center the fixture on the ceiling tile with the light part of the fixture setting on the tile the way it will be positioned when it is installed.
Trace completely around the fixture with a pencil.
Score the pencil marking on the ceiling tile using a utility knife. Make several more passes with the knife until you have cut through the tile completely. Take your time and ensure that you keep the knife on the lines you drew.